Add "b4 shazam" that is like b4 am + git am

By popular demand, provide a way to apply series straight to a git
repository. By default, we're still going the safest possible route:

- create a sparse worktree consisting just of the files being modified
- run "git am" against the temporary worktree
- if "git am" went well, fetch from the temporary worktree into our
  current tree and leave everything in FETCH_HEAD
- unless we're running "b4 shazam -A" in which case we just apply to the
  current HEAD (exact equivalent of b4 am -o- | git am)

Further changes to come based on feedback.

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
3 files changed