ui: use try..finally in configoverride
@contextmanager almost always have their "yield" inside a try..finally
block. This is because if the calling code inside the activated
context manager raises, the code after the "yield" won't get
executed. A "finally" block, however, will get executed in this
scenario.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).