WIP: Add more hooks

This PR extends the hooks in TortoiseGit. I.e. it adds pre, post commit and pre, post pull hooks. Also it changes the behavior of pre and post push to be the same as the already mentioned (i.e., pre push runs immediately before the push starts instead of when the push dialog is opened - in syncdlg it already was correct and post push also runs in case of failure). We differ to TSVN in one case as I'd like to allow the pre commit hook to be able to modify the commit message (for https://code.google.com/p/tortoisegit/issues/detail?id=1605).

Do we also need any other hooks (cf. https://code.google.com/p/tortoisegit/issues/detail?id=1530)? General opinions?