grep: grep the working copy faster `hg grep qqqq` in the mercurial repo: before: 0,859s after: 0,233s `hg grep somethingwithnomatch` in mozilla-central: before: 51s after: 19s 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 behavior. Differential Revision: https://phab.mercurial-scm.org/D8545

grep: stop computing information for --diff when unnecessary This is one reason why `hg grep pattern` essentially does `hg cat -r . 'set:**'` inside. There is no speed improvement in this commit, because the rest of the code still greps data from filelog instead of working copy when possible. Differential Revision: https://phab.mercurial-scm.org/D8544

grep: don't go in an infinite loop when given empty regex Differential Revision: https://phab.mercurial-scm.org/D8543

grep: improve test coverage Differential Revision: https://phab.mercurial-scm.org/D8542