Cannot diff git branch

1. Open a file from a git repository.
2. Select "Tools" -> "Git" -> "Local Repository" -> "Branches..." from the menu.
   Creator's right sidebar appears, showing the list of branches.
3. Right-click on a branch.
4. Select "Diff" from the context menu.
   Creator's diff viewer will show "Retrieving data failed." In the "Version Control" view, you can read:

   Cannot run "git rev-parse --git-dir" in "": fatal: not a git repository (or any parent up to mount point /)
   Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
   
   17:49:55 Running: /usr/bin/git -c diff.color=false diff -m -M -C --first-parent --unified=3 --src-prefix=a/ --dst-prefix=b/ refs/remotes/google/cfl-local-filestore
   error: unknown switch `m'
   usage: git diff --no-index [<options>] <path> <path>
   
   (...) [list of allowed options]
   
   The command "/usr/bin/git -c diff.color=false diff -m -M -C --first-parent --unified=3 --src-prefix=a/ --dst-prefix=b/ refs/remotes/google/cfl-local-filestore" terminated with exit code 129.
   

The shown list of options does not contain "-m", but the same command succeeds when showing a diff against a revision from a log. So the actual problem seems to be that "git diff" is being executed in the wrong working directory.