scmutil: speed up relativization of paths when it's a no-op
Running commands from the root is commmon, in particular for
automation. Running `hg files > /tmp/a` from the root of
mozilla-central on linux:
before:
real 0m1,510s
user 0m1,387s
sys 0m0,090s
after:
real 0m1,266s
user 0m1,165s
sys 0m0,073s
(there are 280k paths, so this was costing ~1us per path somehow)
Differential Revision: https://phab.mercurial-scm.org/D8585
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).