Issue with update.py

[pbelmans]

There is a logic issue with a not too common update scenario. A trivial workaround is to just start with a new database (similar to how one sometimes has to delete temporary files when compiling LaTeX), but having a fully functional update.py would of course be better. Let me quote Johan, describing the error:

First of all, I think the breakage came from either of the following two commits

stacks/stacks-project@09ab84a
stacks/stacks-project@411141e

Note how in the second of these two commits we moved a lemma to a
different file and we changed the tags/tags file to correspond to it
all at the same time. In the first we relabeled some lemmas and we
changed their corresponding lines in the tags file also in the same
commit.

I think there is something in the logic of "update.py" that doesn't
work when you do this. This kind of breakage has happened before and
indeed starting with a new/empty database file is the solution...