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.
#require clang-format test-repo
$ . "$TESTDIR/helpers-testrepo.sh"
$ cd "$TESTDIR"/..
$ for f in `testrepohg files 'set:(**.c or **.cc or **.h) and not "listfile:contrib/clang-format-ignorelist"'` ; do
> clang-format --style file $f > $f.formatted
> cmp $f $f.formatted || diff -u $f $f.formatted
> rm $f.formatted
> done