Improve performance of the git vcs segment

Two changes were made:

1) Fix a bug in the inotify tree watcher that incorrectly marked a tree
as unchanged if an ignored event happens after a non-ignored event. This
allows an optimisation in the git backed to be used (ignoring changes to
.git/index.lock)

2) Get rid of the pygit2 based backend as it is both more complex and
slower than just calling git.