perf: add a `perfbranchmapupdate` command
This command benchmark the time necessary to update the branchmap between two
sets of revisions. This changeset introduce a first version, doing nothing fancy
regarding cache or other internal details.
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2