rebase -i: fix core.commentchar regression

9a46c25bdbf79744 (rebase: interactive: fix short SHA-1 collision,
2013-08-12) added transform_todo_ids() which assumes incorrectly that
the comment character is always '#', however, this has not been the case
since 180bad3d10fe3a7f (rebase -i: respect core.commentchar, 2013-02-11)
patched rebase -i to respect core.commentchar. Fix this regression.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>