grep: move match and diff logic to new module
commands.grep() has lots of functions and classes. Let's split it into
reusable components so we can leverage them to implement a revset predicate
for 'hg grep --diff'. I want to do 'hg log -r "diff(pattern)"'.
test-convert-git.t
test-subrepo-git.t
test-patchbomb-tls.t