merge: add debugmergestate support for _stateextras Now that we can store extras for each file, we need to have support for showing it in debugmergestate (the tests depend on this).

merge: add state extras merge state data In future commits we will want to store more data related to each file in the merge state. This patch adds an optional record for storing a dictionary of extras for each file.

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'.