commit: --squash option for use with rebase --autosquash

commit: --squash option for use with rebase --autosquash

This option makes it convenient to construct commit messages for use
with 'rebase --autosquash'.  The resulting commit message will be
"squash! ..." where "..." is the subject line of the specified commit
message.  This option can be used with other commit message options
such as -m, -c, -C and -F.

If an editor is invoked (as with -c or -eF or no message options) the
commit message is seeded with the correctly formatted subject line.

Example usage:
  $ git commit --squash HEAD~2
  $ git commit --squash HEAD~2 -m "clever comment"
  $ git commit --squash HEAD~2 -F msgfile
  $ git commit --squash HEAD~2 -C deadbeef

Signed-off-by: Pat Notz <patnotz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>