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)
$ cat >> $HGRCPATH << EOF
> [extensions]
> fastannotate=
> EOF
$ hg init repo
$ cd repo
changes to whitespaces
$ cat >> a << EOF
> 1
>
>
> 2
> EOF
$ hg commit -qAm '1'
$ cat > a << EOF
> 1
>
> 2
>
>
> 3
> EOF
$ hg commit -m 2
$ hg fastannotate -wB a
0: 1
0:
1: 2
0:
1:
1: 3