grep: grep the working copy faster
hg grep qqqq in the mercurial repo:
hg grep somethingwithnomatch in mozilla-central:
This is probably also a tiny bug fix, because the code was looking up
a node for filename pfn on a filelog for filename fn, which are
most of the time the same filename, but don't have to be.
Ignoring performance and the bug fix, the code should have the same
Differential Revision: https://phab.mercurial-scm.org/D8545