rebase -i: demonstrate bugs with fixup!/squash! commit messages

rebase -i: demonstrate bugs with fixup!/squash! commit messages

When multiple fixup/squash commands are processed and the last one
causes merge conflicts and is skipped, we leave the "This is a
combination of ..." comments in the commit message.

Noticed by Eric Sunshine.

This regression test also demonstrates that we rely on the localized
version of

	# This is a combination of <number> commits

to contain the <number> in ASCII, which breaks under GETTEXT_POISON.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>