copies: avoid unnecessary copying of copy dict
When storing copy information in the changesets, this patch speeds up
hg debugpathcopies FENNEC_58_0_2_BUILD1 FIREFOX_59_0b8_BUILD2
from 11s to 5.9s. That command takes 6.2s when storing copy
information in filelogs.
Differential Revision: https://phab.mercurial-scm.org/D6421
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