rebase: remember fixup -c after skipping fixup/squash

rebase: remember fixup -c after skipping fixup/squash

When the final command in a chain of "fixup" and "squash" commands
is skipped, we should prompt the user to edit the commit message
if the chain contains a "fixup -c" command that was not skipped.
Unfortunately, commit_staged_changes() only looks for completed "squash"
commands and so does not prompt the user to edit the message. Fix
this by recording whether a fixup command has the "-c" flag set and
then checking whether we have seen either a "fixup -c" or a "squash"
command. Add regression tests for skipping a command in the middle
of the chain (which currently works but has no test coverage), and
for skipping the final command (which is fixed by this patch).

Signed-off-by: Phillip Wood <redacted>
Signed-off-by: Junio C Hamano <redacted>