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