rebase -i: comment out squash!/fixup! subjects from squash message

rebase -i: comment out squash!/fixup! subjects from squash message

When squashing commit messages the squash!/fixup! subjects are not of
interest so comment them out to stop them becoming part of the final
message.

This change breaks a bunch of --autosquash tests which rely on the
"squash! <subject>" line appearing in the final commit message. This is
addressed by adding a second line to the commit message of the "squash!
..." commits and testing for that.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Charvi Mendiratta <charvi077@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>