[MERGE] Add tests for start_commit hook.

[Alexander Belchenko]

Jelmer Vernooij пишет:
> This patch adds a start_commit hook for MutableTrees (bug 186422). This
> hook is executed before a commit is started and can actually modify the
> state of the working tree, for example by adding files. This patch is
> required to be able to support bzr in etckeeper
> (http://kitenet.net/~joey/code/etckeeper/)

bb:approve