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)
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..