update_weblate_git.py: don't create merge commits when we could fast-forward

That is, after having prepared the index for the merge commit, if “git diff —cached remote” is empty, i.e. if we haven’t done anything on top of applying the changes from the remote branch, then drop everything and fast-forward merge.

Goal: don’t create tons of unneeded merge commits in our Git history.

Parent Task: #15082 (closed)

Original created by @intrigeri on 16842 (Redmine)