rebase -i: fix parsing of "fixup -C<commit>"

rebase -i: fix parsing of "fixup -C<commit>"

If the user omits the space between "-C" and the commit in a fixup
command then it is parsed as an ordinary fixup and the commit message is
not updated as it should be. Fix this by making the space between "-C"
and "<commit>" optional as it is for the "merge" command.

Note that set_replace_editor() is changed to set $GIT_SEQUENCE_EDITOR
instead of $EDITOR in order to be able to replace the todo list and
reword commits with $FAKE_COMMIT_MESSAGE. This is safe as all the
existing users are using set_replace_editor() to replace the todo list.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>