git: fix windows gitfile paths

Before

    gitdir: \\?\C:\Users\runneradmin\AppData\Local\Temp\jj-test-fy26a2\second

After

    gitdir: C:/Users/runneradmin/AppData/Local/Temp/jj-test-fy26a2/second

And same for the other worktree bits and pieces. We also change the way
the tests run git commands, because of yet another error to do with
Git being unable to handle verbatim paths:

    command=`"git" "worktree" "add" "\\\\?\\C:\\Users\\runneradmin\\AppData\\Local\\Temp\\jj-test-ti0MHE\\second"`
    code=128
    stdout=""
    stderr=```
    Preparing worktree (new branch \'second\')
    fatal: could not create leading directories of \'//?/C:/Users/runneradmin/AppData/Local/Temp/jj-test-ti0MHE/second/.git\': Invalid argument

cli/tests/test_git_colocated.rs

@@ -1028,24 +1028,21 @@ fn test_colocated_workspace_fail_existing_git_worktree() {
 
     let test_env = TestEnvironment::default();
     let repo_path = test_env.env_root().join("repo");
-    let second_path = test_env.env_root().join("second");
     let third_path = test_env.env_root().join("third");
 
     // Initialize and make a commit so git can create worktrees
     test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "--colocate", "repo"]);
     std::fs::write(repo_path.join("file.txt"), "contents").unwrap();
     test_env.jj_cmd_ok(&repo_path, &["commit", "-m", "initial commit"]);
 
-    // Add a a git worktree at `.git/worktrees/second` that points to "../third"
+    // Add a git worktree at `.git/worktrees/second` that points to "../third"
     Command::new("git")
-        .args(["worktree", "add"])
-        .arg(&second_path)
+        .args(["worktree", "add", "../second"])
         .current_dir(&repo_path)
         .assert()
         .success();
     Command::new("git")
-        .args(["worktree", "move", "second"])
-        .arg(&third_path)
+        .args(["worktree", "move", "second", "../third"])
         .current_dir(&repo_path)
         .assert()
         .success();

lib/src/git.rs

@@ -269,6 +269,96 @@ impl CreateWorktreeError {
     }
 }
 
+pub fn format_gitfile_path(path: &Path) -> Cow<'_, str> {
+    return to_slash_lossy(path);
+
+    // This is from https://docs.rs/path-slash (MIT licensed)
+    // but tweaked to change any verbatim prefix to non-verbatim, as git does.
+    //
+    // Copyright (c) 2018 rhysd
+    //
+    // Permission is hereby granted, free of charge, to any person obtaining a copy
+    // of this software and associated documentation files (the "Software"), to deal
+    // in the Software without restriction, including without limitation the rights
+    // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    // copies of the Software, and to permit persons to whom the Software is
+    // furnished to do so, subject to the following conditions:
+    //
+    // The above copyright notice and this permission notice shall be included in
+    // all copies or substantial portions of the Software.
+    //
+    // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+    // THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
+    // OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+    // ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+    // OTHER DEALINGS IN THE SOFTWARE.
+    //
+
+    #[cfg(target_os = "windows")]
+    mod windows {
+        use std::os::windows::ffi::OsStrExt as _;
+        use std::path::MAIN_SEPARATOR;
+
+        use super::*;
+
+        // Workaround for Windows. There is no way to extract raw byte sequence from
+        // `OsStr` (in `Path`). And `OsStr::to_string_lossy` may cause extra
+        // heap allocation.
+        pub(crate) fn ends_with_main_sep(p: &Path) -> bool {
+            p.as_os_str().encode_wide().last() == Some(MAIN_SEPARATOR as u16)
+        }
+    }
+    #[cfg(not(target_os = "windows"))]
+    fn to_slash_lossy(path: &Path) -> Cow<'_, str> {
+        path.to_string_lossy()
+    }
+    #[cfg(target_os = "windows")]
+    fn to_slash_lossy(path: &Path) -> Cow<'_, str> {
+        use std::path::Component;
+        use std::path::Prefix;
+
+        let mut buf = String::new();
+        for c in path.components() {
+            match c {
+                Component::RootDir => { /* empty */ }
+                Component::CurDir => buf.push('.'),
+                Component::ParentDir => buf.push_str(".."),
+                Component::Prefix(prefix_component) => {
+                    match prefix_component.kind() {
+                        Prefix::Disk(disk) | Prefix::VerbatimDisk(disk) => {
+                            if let Some(c) = char::from_u32(disk as u32) {
+                                buf.push(c);
+                                buf.push(':');
+                            }
+                        }
+                        Prefix::UNC(host, share) | Prefix::VerbatimUNC(host, share) => {
+                            // Write it as non-verbatim but with two forward slashes // instead of
+                            // \\. (Second slash is written below.) I
+                            // think this sounds right? https://learn.microsoft.com/en-us/dotnet/core/compatibility/core-libraries/5.0/unc-path-recognition-unix
+                            buf.push('/')
+                        }
+                        // Just ignore it and hope for the best?
+                        Prefix::Verbatim(_) => {}
+                        Prefix::DeviceNS(_) => {}
+                    }
+                    // C:\foo is [Prefix, RootDir, Normal]. Avoid C://
+                    continue;
+                }
+                Component::Normal(s) => buf.push_str(&s.to_string_lossy()),
+            }
+            buf.push('/');
+        }
+
+        if !windows::ends_with_main_sep(path) && buf != "/" && buf.ends_with('/') {
+            buf.pop(); // Pop last '/'
+        }
+
+        Cow::Owned(buf)
+    }
+}
+
 /// `git worktree add` implementation
 ///
 /// This function has to be implemented from scratch.
@@ -318,12 +408,12 @@ pub fn git_worktree_add(
 
     {
         let mut commondir_file = file_create_new(worktree_data.join("commondir"))?;
-        commondir_file.write_all(Path::new("..").join("..").as_os_str().as_encoded_bytes())?;
+        commondir_file.write_all(format_gitfile_path(&Path::new("..").join("..")).as_bytes())?;
         writeln!(commondir_file)?;
     }
     {
         let mut gitdir_file = file_create_new(gitdir_file_path)?;
-        gitdir_file.write_all(worktree_dotgit_path.as_os_str().as_encoded_bytes())?;
+        gitdir_file.write_all(format_gitfile_path(&worktree_dotgit_path).as_bytes())?;
         writeln!(gitdir_file)?;
     }
 
@@ -343,7 +433,7 @@ pub fn git_worktree_add(
     {
         let mut dot_git = file_create_new(&worktree_dotgit_path)?;
         write!(dot_git, "gitdir: ")?;
-        dot_git.write_all(worktree_data.as_os_str().as_encoded_bytes())?;
+        dot_git.write_all(format_gitfile_path(&worktree_data).as_bytes())?;
         writeln!(dot_git)?;
     }