b4: introducing b4 shazam (like b4 am -o- | git am)

[Konstantin Ryabitsev <konstantin@linuxfoundation.org>]

Hi, all:

The current master branch landed a new feature that implements a new
subcommand "shazam". It is the functional equivalent of piping "b4 am" to "git
am", with some extra safety padding.

See "b4 shazam --help" to see the available options. You will notice that many
of them are exactly the same as for "b4 am".

The default operation still tries to be the safest possible:

- we'll create a temporary sparse worktree that will only contain the files
  being modified
- we'll use the base-commit sha, if we find it, or we'll try to guess the base
  commit from git index info
- we'll run "git am" against that temporary worktree
- if successful, we'll fetch from the worktree into your current repo and
  delete the temporary sparse checkout
- you can then "git checkout -b foo FETCH_HEAD" or "git merge FETCH_HEAD"
  or "git rebase" as necessary

Alternatively, if you run "git shazam -A", we'll just run "git am" on the
current HEAD, which is the exact equivalent of "b4 am -o- | git am".

This feature is fairly "raw" and lightly tested, so please expect things to
potentially blow up all over the place. :)

If the default operation with getting things ready in FETCH_HEAD is not
useful, please let me know how you would prefer to see things work instead.

Best regards,
-K