perf: fix the time measurement for pathcopies relative to p2
PyCharm flagged this as unused. But it seems more likely that it was intended
to reset the time for the p2 path.
Differential Revision: https://phab.mercurial-scm.org/D7742
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.