shears.sh: support fixup! and squash!
One of the most powerful feature added to the already powerful
interactive rebase was to allow workflows where you *add* commits with
the intention to amend ("fix up") other commits in the topic branch at
a later stage, through "git rebase -i --autosquash". Such commits are
added by crafting special-format commit messages, or by using the
convenient --fixup or --squash options of "git commit".
However, with the garden shears, we side-step the original interactive
rebase, therefore we have to re-implement that very convenient
fixup/squash handling.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>