cli: add --second to split command

CHANGELOG.md

@@ -41,6 +41,9 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * Templates now support the `>=`, `>`, `<=`, and `<` relational operators for
   `Integer` types.
 
+* `jj split` now accepts `--second` to put specified paths into the second
+  commit instead of the first.
+
 ### Fixed bugs
 
 * The `$NO_COLOR` environment variable must now be non-empty to be respected.

cli/src/commands/split.rs

@@ -15,6 +15,8 @@ use std::io::Write;
 
 use clap_complete::ArgValueCandidates;
 use clap_complete::ArgValueCompleter;
+use jj_lib::matchers::DifferenceMatcher;
+use jj_lib::matchers::EverythingMatcher;
 use jj_lib::object_id::ObjectId;
 use jj_lib::repo::Repo;
 use tracing::instrument;
@@ -66,12 +68,16 @@ pub(crate) struct SplitArgs {
     // TODO: Delete `--siblings` alias in jj 0.25+
     #[arg(long, short, alias = "siblings")]
     parallel: bool,
-    /// Put these paths in the first commit
+    /// Put these paths into one commit (the first by default).
     #[arg(
         value_hint = clap::ValueHint::AnyPath,
         add = ArgValueCompleter::new(complete::modified_revision_files),
     )]
     paths: Vec<String>,
+
+    /// Put the specified paths into the second commit instead of the first.
+    #[arg(long, short, conflicts_with_all = ["parallel", "interactive", "tool"])]
+    second: bool,
 }
 
 #[instrument(skip_all)]
@@ -90,9 +96,13 @@ pub(crate) fn cmd_split(
     }
 
     workspace_command.check_rewritable([commit.id()])?;
-    let matcher = workspace_command
+    let mut matcher = workspace_command
         .parse_file_patterns(ui, &args.paths)?
         .to_matcher();
+    if args.second {
+        matcher = Box::new(DifferenceMatcher::new(EverythingMatcher, matcher));
+    }
+
     let diff_selector = workspace_command.diff_selector(
         ui,
         args.tool.as_deref(),

cli/tests/cli-reference@.md.snap

@@ -2058,7 +2058,7 @@ Splitting an empty commit is not supported because the same effect can be achiev
 
 ###### **Arguments:**
 
-* `<PATHS>` — Put these paths in the first commit
+* `<PATHS>` — Put these paths into one commit (the first by default)
 
 ###### **Options:**
 
@@ -2068,6 +2068,7 @@ Splitting an empty commit is not supported because the same effect can be achiev
 
   Default value: `@`
 * `-p`, `--parallel` — Split the revision into two parallel revisions instead of a parent and child
+* `-s`, `--second` — Put the specified paths into the second commit instead of the first
 
 
 

cli/tests/test_split_command.rs

@@ -150,6 +150,25 @@ fn test_split_by_paths() {
     insta::assert_snapshot!(stdout, @r###"
     A file2
     "###);
+
+    let stdout = test_env.jj_cmd_success(&repo_path, &["diff", "-s", "-r", "@"]);
+    insta::assert_snapshot!(stdout, @r###"
+    A file1
+    A file3
+    "###);
+
+    let (stdout, _) = test_env.jj_cmd_ok(&repo_path, &["split", "--second", "file3"]);
+    insta::assert_snapshot!(stdout, @"");
+
+    let stdout = test_env.jj_cmd_success(&repo_path, &["diff", "-s", "-r", "@-"]);
+    insta::assert_snapshot!(stdout, @r###"
+    A file1
+    "###);
+
+    let stdout = test_env.jj_cmd_success(&repo_path, &["diff", "-s", "-r", "@"]);
+    insta::assert_snapshot!(stdout, @r###"
+    A file3
+    "###);
 }
 
 #[test]