rebase: Resolve revisionExpr to tracking branch for --onto-manifest

When running `repo rebase -m` (`--onto-manifest`), the command uses the
raw `revisionExpr` from the manifest (e.g. `main`) directly as the
`--onto` target. This can fail or behave incorrectly if it should
reference the local tracking branch (e.g. `refs/remotes/<remote>/main`).

Resolve `project.revisionExpr` to its local tracking branch using
`project.GetRemote().ToLocal()`. Fall back to using the raw
`revisionExpr` value if the resolution fails (raising a `GitError`).

Bug: 532028666
Change-Id: I4c1bca1374a5842688be227f6aa2afffcdad5397
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/604941
Reviewed-by: Brian Gan <brgan@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

subcmds/rebase.py

@@ -16,7 +16,9 @@ import sys
 
 from color import Coloring
 from command import Command
+from error import GitError
 from git_command import GitCommand
+from project import Project
 from repo_logging import RepoLogger
 
 
@@ -30,6 +32,17 @@ class RebaseColoring(Coloring):
         self.fail = self.printer("fail", fg="red")
 
 
+def _ResolveOntoManifest(project: Project) -> str:
+    """Resolve project's revisionExpr to a local tracking branch.
+
+    Falls back to the raw revisionExpr if ToLocal fails or raises GitError.
+    """
+    try:
+        return project.GetRemote().ToLocal(project.revisionExpr)
+    except GitError:
+        return project.revisionExpr
+
+
 class Rebase(Command):
     COMMON = True
     helpSummary = "Rebase local branches on upstream branch"
@@ -162,7 +175,7 @@ branch but need to incorporate new upstream changes "underneath" them.
             args = common_args[:]
             if opt.onto_manifest:
                 args.append("--onto")
-                args.append(project.revisionExpr)
+                args.append(_ResolveOntoManifest(project))
 
             args.append(upbranch.LocalMerge)