VCS[git]: Add support for stashes.
Add non-modal stash management dialog and additional menu option "Stash snapshot..." to stash away changes prompting for a description, which will immediately replay the stash (take snapshot and continue working). Add interface to IVersionControl for creating/restoring/deleting snapshots for backup/complex undo operations (currently supported by git only). Add test options to VCSBasePlugin. Clean up and extend git client accordingly.