"Error during git operations", due to shared ref file leading to `~1` file names

Reported by @sponce and seen in lhcb/LHCb!3392 (merged)

There are three tests which all use the same reference file.
A diff in this ref, even if it is the same diff for all 3, will lead to 3 new ref files:

Det/DetCond/tests/refs/begin_event_no_reserve.ref
Det/DetCond/tests/refs/begin_event_no_reserve.ref.~1~
Det/DetCond/tests/refs/begin_event_no_reserve.ref.~2~

The error that will lead to the abort is below:

WARNING [update_references.py::212] LHCb: Download contains additinal ref files we didn't expect: {'Det/DetCond/tests/refs/begin_event_no_reserve.ref.~1~',
 'Det/DetCond/tests/refs/begin_event_no_reserve.ref.~2~'}
The following paths are ignored by one of your .gitignore files:
Det/DetCond/tests/refs/begin_event_no_reserve.ref.~1~
Det/DetCond/tests/refs/begin_event_no_reserve.ref.~2~
hint: Use -f if you really want to add them.
hint: Turn this message off by running
hint: "git config advice.addIgnoredFile false"

The refbot should probably be able to handle such a scenario, but it would also need to check that all three ref updates in this case are identical.