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scmutil: speed up relativization of paths when it's a no-op

Description

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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