rebase -i --autosquash: auto-squash commits

rebase -i --autosquash: auto-squash commits

Teach a new option, --autosquash, to the interactive rebase.
When the commit log message begins with "!fixup ...", and there
is a commit whose title begins with the same ..., automatically
modify the todo list of rebase -i so that the commit marked for
squashing come right after the commit to be modified, and change
the action of the moved commit from pick to squash.

Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>