commands: move rebase_to_dest_parent to jj_lib::rewrite::rebase_source

What make rebase_to_dest_parent a good candidate for jj_lib::rewrite module:

- It is used both in obslog and interdiff. It's a sign that it may be moved to a lower layer
- CommandError is returned by converting from TreeMergeError. Not explicitly.
- It only use jj_lib::rewrite fonctions.

cli/src/commands/interdiff.rs

@@ -13,9 +13,9 @@
 // limitations under the License.
 
 use clap::ArgGroup;
+use jj_lib::rewrite::rebase_source;
 use tracing::instrument;
 
-use super::rebase_to_dest_parent;
 use crate::cli_util::{CommandError, CommandHelper, RevisionArg};
 use crate::diff_util::{self, DiffFormatArgs};
 use crate::ui::Ui;
@@ -51,7 +51,7 @@ pub(crate) fn cmd_interdiff(
     let from = workspace_command.resolve_single_rev(args.from.as_deref().unwrap_or("@"), ui)?;
     let to = workspace_command.resolve_single_rev(args.to.as_deref().unwrap_or("@"), ui)?;
 
-    let from_tree = rebase_to_dest_parent(&workspace_command, &from, &to)?;
+    let from_tree = rebase_source(workspace_command.repo().as_ref(), &from, &to)?;
     let to_tree = to.tree()?;
     let matcher = workspace_command.matcher_from_values(&args.paths)?;
     let diff_formats = diff_util::diff_formats_for(command.settings(), &args.format)?;

cli/src/commands/mod.rs

@@ -63,9 +63,7 @@ use clap::{Command, CommandFactory, FromArgMatches, Subcommand};
 use itertools::Itertools;
 use jj_lib::commit::Commit;
 use jj_lib::matchers::EverythingMatcher;
-use jj_lib::merged_tree::MergedTree;
 use jj_lib::repo::ReadonlyRepo;
-use jj_lib::rewrite::merge_commit_trees;
 use jj_lib::settings::UserSettings;
 use tracing::instrument;
 
@@ -147,24 +145,6 @@ enum Commands {
     Workspace(workspace::WorkspaceCommands),
 }
 
-fn rebase_to_dest_parent(
-    workspace_command: &WorkspaceCommandHelper,
-    source: &Commit,
-    destination: &Commit,
-) -> Result<MergedTree, CommandError> {
-    if source.parent_ids() == destination.parent_ids() {
-        Ok(source.tree()?)
-    } else {
-        let destination_parent_tree =
-            merge_commit_trees(workspace_command.repo().as_ref(), &destination.parents())?;
-        let source_parent_tree =
-            merge_commit_trees(workspace_command.repo().as_ref(), &source.parents())?;
-        let source_tree = source.tree()?;
-        let rebased_tree = destination_parent_tree.merge(&source_parent_tree, &source_tree)?;
-        Ok(rebased_tree)
-    }
-}
-
 fn edit_description(
     repo: &ReadonlyRepo,
     description: &str,

cli/src/commands/obslog.rs

@@ -15,9 +15,9 @@
 use jj_lib::commit::Commit;
 use jj_lib::dag_walk::topo_order_reverse;
 use jj_lib::matchers::EverythingMatcher;
+use jj_lib::rewrite::rebase_source;
 use tracing::instrument;
 
-use super::rebase_to_dest_parent;
 use crate::cli_util::{
     CommandError, CommandHelper, LogContentFormat, RevisionArg, WorkspaceCommandHelper,
 };
@@ -153,7 +153,7 @@ fn show_predecessor_patch(
         Some(predecessor) => predecessor,
         None => return Ok(()),
     };
-    let predecessor_tree = rebase_to_dest_parent(workspace_command, predecessor, commit)?;
+    let predecessor_tree = rebase_source(workspace_command.repo().as_ref(), predecessor, commit)?;
     let tree = commit.tree()?;
     diff_util::show_diff(
         ui,

lib/src/rewrite.rs

@@ -127,6 +127,24 @@ pub fn rebase_commit(
         .write()?)
 }
 
+pub fn rebase_source(
+    repo: &dyn Repo,
+    source: &Commit,
+    destination: &Commit,
+) -> Result<MergedTree, TreeMergeError> {
+    if source.parent_ids() == destination.parent_ids() {
+        Ok(source.tree()?)
+    } else {
+        let destination_parent_tree =
+            merge_commit_trees(repo, &destination.parents())?;
+        let source_parent_tree =
+            merge_commit_trees(repo, &source.parents())?;
+        let source_tree = source.tree()?;
+        let rebased_tree = destination_parent_tree.merge(&source_parent_tree, &source_tree)?;
+        Ok(rebased_tree)
+    }
+}
+
 pub fn back_out_commit(
     settings: &UserSettings,
     mut_repo: &mut MutableRepo,