The revision walking code made the PBGitRepository unclean. Especially if
we want to keep multiple PBGitRepository objects around (e.g. persistent
data store), it needs to be more simple. This neatly extracts the revision
walking code from the repository code.
This makes the initial startup much faster. Commits get loaded in a separate
thread, and are displayed every 1000 commits.
There is a bug in here that makes fails a click while it is loading the commit
list. Not sure how to fix this, perhaps send the arrayController?
Pieter de Bie