operations: Fix error when rebase in UserSquash fails
With the UserSquashWithoutWorktree flag, we're rebasing commits on top of the start commit before doing the actual squash. But when the rebase fails because of a conflicts, then we do not properly return a proper Git error like we did before, when patch application on top of the start commit failed.
Fix the issue by returning a gitError
such that we return the correct
response.
Changelog: fixed
Part of #3713 (closed)