jj rebase: Allow revsets with multiple commits with new --multi

Eventually, we should be able to rely on `jj op restore` and the `--multi`
argument should likely be removed. This is a temporary measure until we figure
out #922 and the like.

Fixes #571

CHANGELOG.md

@@ -106,6 +106,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   and `jj split any-non-existent-path` inserts an empty commit between the
   target commit and its parents.
 
+* `jj rebase` now accepts a new `--multi` argument that allows the revset in the
+  `-d` argument to expand to several revisions. For example, `jj rebase -s B -d
+  B- -d C` now works even if `B` is a merge commit.
+
 ### Fixed bugs
 
 * When sharing the working copy with a Git repo, we used to forget to export

src/cli_util.rs

@@ -1386,15 +1386,26 @@ fn load_revset_aliases(
 pub fn resolve_base_revs(
     workspace_command: &WorkspaceCommandHelper,
     revisions: &[RevisionArg],
+    multi: bool,
 ) -> Result<IndexSet<Commit>, CommandError> {
     let mut commits = IndexSet::new();
-    for revision_str in revisions {
-        let commit = workspace_command.resolve_single_rev(revision_str)?;
-        let commit_hash = short_commit_hash(commit.id());
-        if !commits.insert(commit) {
-            return Err(user_error(format!(
-                r#"More than one revset resolved to revision {commit_hash}"#,
-            )));
+    if multi {
+        for revset in revisions {
+            let revisions = workspace_command.resolve_revset(revset)?;
+            workspace_command.check_non_empty(&revisions)?;
+            // It's OK if one of the base revs includes a non-rewritable commit
+            commits.extend(revisions);
+        }
+    } else {
+        // TODO: Change the error resolve_single_rev returns!
+        for revision_str in revisions {
+            let commit = workspace_command.resolve_single_rev(revision_str)?;
+            let commit_hash = short_commit_hash(commit.id());
+            if !commits.insert(commit) {
+                return Err(user_error(format!(
+                    r#"More than one revset resolved to revision {commit_hash}"#,
+                )));
+            }
         }
     }
 

src/commands/mod.rs

@@ -444,6 +444,9 @@ struct NewArgs {
     /// Parent(s) of the new change
     #[arg(default_value = "@")]
     revisions: Vec<RevisionArg>,
+    /// Allow revsets expanding to multiple commits in a single argument
+    #[arg(long)]
+    multi: bool,
     /// Ignored (but lets you pass `-r` for consistency with other commands)
     #[arg(short = 'r', hide = true)]
     unused_revision: bool,
@@ -725,6 +728,9 @@ struct RebaseArgs {
     /// The revision(s) to rebase onto
     #[arg(long, short, required = true)]
     destination: Vec<RevisionArg>,
+    /// Allow revsets expanding to multiple commits in a single argument
+    #[arg(long)]
+    multi: bool,
 }
 
 /// Apply the reverse of a revision on top of another revision
@@ -1994,7 +2000,7 @@ fn cmd_new(ui: &mut Ui, command: &CommandHelper, args: &NewArgs) -> Result<(), C
         !args.revisions.is_empty(),
         "expected a non-empty list from clap"
     );
-    let commits = resolve_base_revs(&workspace_command, &args.revisions)?
+    let commits = resolve_base_revs(&workspace_command, &args.revisions, args.multi)?
         .into_iter()
         .collect_vec();
     let parent_ids = commits.iter().map(|c| c.id().clone()).collect();
@@ -2685,7 +2691,7 @@ fn cmd_merge(ui: &mut Ui, command: &CommandHelper, args: &NewArgs) -> Result<(),
 
 fn cmd_rebase(ui: &mut Ui, command: &CommandHelper, args: &RebaseArgs) -> Result<(), CommandError> {
     let mut workspace_command = command.workspace_helper(ui)?;
-    let new_parents = resolve_base_revs(&workspace_command, &args.destination)?
+    let new_parents = resolve_base_revs(&workspace_command, &args.destination, args.multi)?
         .into_iter()
         .collect_vec();
     if let Some(rev_str) = &args.revision {

tests/test_rebase_command.rs

@@ -321,6 +321,17 @@ fn test_rebase_multiple_destinations() {
     fe2e8e8b50b3 c
     d370aee184ba b
     "###);
+    let stdout =
+        test_env.jj_cmd_success(&repo_path, &["rebase", "--multi", "-r", "a", "-d", "b|c"]);
+    insta::assert_snapshot!(stdout, @r###""###);
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    o   a
+    |\  
+    @ | c
+    | o b
+    |/  
+    o 
+    "###);
 
     let stderr =
         test_env.jj_cmd_failure(&repo_path, &["rebase", "-r", "a", "-d", "b", "-d", "root"]);