revset: use manifest.matches in _follow revset The old _follow revset iterated over every file in the commit and checked if it matched. For repos with large manifests, this could take 500ms. By switching to use manifest.matches() we can take advantage of the fastpaths built in to manifest.py that allows iterating over only the files in the matcher when it's a simple matcher. This brings the time spent down from 500ms to 0ms during simple operations like 'hg log -f file.txt'.