(For CI -- test that the parent PR works)

Cargo.toml

@@ -48,7 +48,7 @@ dunce = "1.0.4"
 either = "1.13.0"
 esl01-renderdag = "0.3.0"
 futures = "0.3.30"
-git2 = { version = "0.18.3", features = [
+git2 = { version = "0.19.0", features = [
     # Do *not* disable this feature even if you'd like dynamic linking. Instead,
     # set the environment variable `LIBGIT2_NO_VENDOR=1` if dynamic linking must
     # be used (this will override the Cargo feature), and allow static linking

cli/Cargo.toml

@@ -74,6 +74,7 @@ jj-lib = { workspace = true }
 maplit = { workspace = true }
 minus = { workspace = true }
 once_cell = { workspace = true }
+path-slash = "0.2.1"
 pest = { workspace = true }
 pest_derive = { workspace = true }
 pollster = { workspace = true }

cli/src/commands/git/clone.rs

@@ -20,6 +20,7 @@ use jj_lib::git::{self, GitFetchError, GitFetchStats};
 use jj_lib::repo::Repo;
 use jj_lib::str_util::StringPattern;
 use jj_lib::workspace::Workspace;
+use path_slash::PathBufExt;
 
 use crate::cli_util::{CommandHelper, WorkspaceCommandHelper};
 use crate::command_error::{cli_error, user_error, user_error_with_message, CommandError};
@@ -47,18 +48,32 @@ pub struct GitCloneArgs {
     colocate: bool,
 }
 
+fn looks_like_a_path_to_git(source: &str) -> bool {
+    source.chars().nth(1) == Some(':') || // Looks like a Windows C:... path
+    // Match Git's behavior, the URL syntax [is only recognized if there are no
+    // slashes before the first
+    // colon](https://git-scm.com/docs/git-clone#_git_urls)
+    !source
+        .split_once("/")  
+        .map_or(source, |(first, _)| first)
+        .contains(":")
+}
+
 fn absolute_git_source(cwd: &Path, source: &str) -> String {
     // Git appears to turn URL-like source to absolute path if local git directory
     // exits, and fails because '$PWD/https' is unsupported protocol. Since it would
     // be tedious to copy the exact git (or libgit2) behavior, we simply assume a
     // source containing ':' is a URL, SSH remote, or absolute path with Windows
     // drive letter.
-    if !source.contains(':') && Path::new(source).exists() {
-        // It's less likely that cwd isn't utf-8, so just fall back to original source.
-        cwd.join(source)
-            .into_os_string()
-            .into_string()
-            .unwrap_or_else(|_| source.to_owned())
+    if looks_like_a_path_to_git(source) && Path::new(source).exists() {
+        // TODO: This won't work for Windows UNC path or (less importantly) if the
+        // original source is a non-UTF Windows path. For Windows UNC paths, we
+        // could use dunce, though see also https://gitlab.com/kornelski/dunce/-/issues/7
+        cwd.join(source).to_slash().map_or_else(
+            // It's less likely that cwd isn't utf-8, so just fall back to original source.
+            || source.to_owned(),
+            |s| s.to_string(),
+        )
     } else {
         source.to_owned()
     }

cli/tests/common/mod.rs

@@ -17,6 +17,7 @@ use std::collections::HashMap;
 use std::path::{Path, PathBuf};
 
 use itertools::Itertools as _;
+use path_slash::PathBufExt;
 use regex::{Captures, Regex};
 use tempfile::TempDir;
 
@@ -320,8 +321,9 @@ impl TestEnvironment {
     pub fn normalize_output(&self, text: &str) -> String {
         let text = text.replace("jj.exe", "jj");
         let regex = Regex::new(&format!(
-            r"{}(\S+)",
-            regex::escape(&self.env_root.display().to_string())
+            r"({}|{})(\S+)",
+            regex::escape(&self.env_root.display().to_string()),
+            regex::escape(&self.env_root.to_slash_lossy())
         ))
         .unwrap();
         regex

lib/src/git.rs

@@ -1265,7 +1265,12 @@ pub fn fetch(
     tracing::debug!("remote.prune");
     remote.prune(None)?;
     tracing::debug!("remote.update_tips");
-    remote.update_tips(None, false, git2::AutotagOption::Unspecified, None)?;
+    remote.update_tips(
+        None,
+        git2::RemoteUpdateFlags::empty(),
+        git2::AutotagOption::Unspecified,
+        None,
+    )?;
     // TODO: We could make it optional to get the default branch since we only care
     // about it on clone.
     let mut default_branch = None;