perf: add a `--clear-caches` to `perfbranchmapupdate`
This flag will help to measure the time we spend loading various cache that
support the branchmap update.
Example for an 500 000 revisions repository:
hg perfbranchmapupdate --base 'not tip' --target 'tip'
! wall 0.000860 comb 0.000000 user 0.000000 sys 0.000000 (best of 336)
hg perfbranchmapupdate --base 'not tip' --target 'tip' --clear-caches
! wall 0.029494 comb 0.030000 user 0.030000 sys 0.000000 (best of 100)
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).