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