rebase --merge: fix for rebasing more than 7 commits.

rebase --merge: fix for rebasing more than 7 commits.

Instead of using 4-digit numbers to name commits being rebased,
just use "cmt.$msgnum" string, with $msgnum as a decimal number
without leading zero padding.  This makes it possible to rebase
more than 9999 commits, but of more practical importance is that
the earlier code used "printf" to format already formatted
$msgnum and barfed when it counted up to 0008.  In other words,
the old code was incapable of rebasing more than 7 commits, and
this fixes that problem.

Signed-off-by: Junio C Hamano <junkio@cox.net>