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
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