rebase-i-p: minimum fix to obvious issues
Jeff King noticed that this series uses non-portable ${var:0:7} syntax
to splice a string, which is not even in POSIX, in the script. A quick
look at around the offending part revealed a few issues, which this commit
fixes:
* Why filter output from "rev-list --left-right A...B" and look for the
ones that begin with ">"? Wouldn't "rev-list A..B" give that?
* The abbreviated SHA-1 are made with "rev-list --abbrev=7" into $TODO in
an earlier invocation, and it can be more than 7 letters to avoid
ambiguity. Not just that "${r:0:7} is not even in POSIX", but use of
it here is actively wrong.
* There is no point in catting a single file and piping it into grep.
Signed-off-by: Junio C Hamano <gitster@pobox.com>