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>

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