git: Reorganize the menu, again
Almost everything, except for Initialize Repository, is placed in a submenu:
- Commit, Diff, Log, and Stash are under Changes
- Push, Pull, and Fetch are under Remote Repository
- Add, Remove, Ignore, Checkout, Unstage, and Reesolve are
under Files
- Rebase has been moved to Branches
- Reset and Revert have their own submenu
With these changes, we should be fully HIG compliant as far as the length of
the menu is concerned (no more than 15 items) with some room to spare.