relprep.py: bump Firefox and GV to a (yet) non-existing tag when the last one does not match HEAD

We almost always want to build releases off HEAD.

Since rarely it happened we forgot to bump the tag to the correct one (e.g., 13.5.6+13.5.7 firefox-android), we thought we could bump the tag to a non-existing one when the last available tag doesn't match HEAD.

Usually, it will happen that we'll tag right after before running the build, and everything will work.

Sometimes, we'll forget, and the build will fail immediately.

In some other times, not building from HEAD was intended, and we can adjust the tag manually before creating the commit.