perf: add a `clear-revlogs` flag to `perftags`
This flag (on by default) makes it possible to disable the refresh of the
changelog and revlog. This is useful to check for the time spent in the core
tags logic without the associated side effects. Usually, these side effects
are shared with other logics (eg: bookmarks).
Example output in my Mercurial repository
$ hg perftags
! wall 0.017919 comb 0.020000 user 0.020000 sys 0.000000 (best of 141)
$ hg perftags --no-clear-revlogs
! wall 0.012982 comb 0.010000 user 0.010000 sys 0.000000 (best of 207)
{
// Enforcing
"eqeqeq" : true, // true: Require triple equals (===) for comparison
"forin" : true, // true: Require filtering for..in loops with obj.hasOwnProperty()
"freeze" : true, // true: prohibits overwriting prototypes of native objects such as Array, Date etc.
"nonbsp" : true, // true: Prohibit "non-breaking whitespace" characters.
"undef" : true, // true: Require all non-global variables to be declared (prevents global leaks)
// Environments
"browser" : true // Web Browser (window, document, etc)
}