perf: add a `clear-revlogs` flag to `perfbookmarks`
This flag (off by default) makes it possible to enable the refresh of the
changelog and revlog. This is useful to check for costly side effects of
bookmark loading.
Usually, these side effects are shared with other logics (eg: tags).
example output in my mercurial repo (with 1 bookmark, so not a great example):
$ hg perfbookmarks
! wall 0.000044
$ hg perfbookmarks --clear-revlogs
! wall 0.001380