Wire branch completion into git diff argument positions (#228)

Co-authored-by: Oz <oz-agent@warp.dev>

Author: lucieleblanc

command-signatures/json/git.json

@@ -4486,6 +4486,8 @@
                     }
                 ],
                 "generatorName": [
+                    "local_branches",
+                    "remote_branches",
                     "commits",
                     "get_changed_or_tracked_files"
                 ],