copies: avoid calling matcher if matcher.always()
When storing copy information in the changesets
(experimental.copies.read-from=changeset-only), this patch speeds up
hg debugpathcopies FENNEC_58_0_2_BUILD1 FIREFOX_59_0b8_BUILD2
from 5.9s to 4.7s. At the start of this series (
b162229e), that
command took 18min.
Differential Revision: https://phab.mercurial-scm.org/D6422
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