rebase-i: clarify "is this commit relevant?" test

While I was checking all the call sites of sane_grep and sane_egrep,
I noticed this one is somewhat strangely written.  The lines in the
file sane_grep works on all begin with 40-hex object name, so there
is no real risk of confusing "test $(...) = ''" by finding something
that begins with a dash, but using the status from sane_grep makes
it a lot clearer what is going on.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -1237,7 +1237,8 @@ then
 	git rev-list $revisions |
 	while read rev
 	do
-		if test -f "$rewritten"/$rev && test "$(sane_grep "$rev" "$state_dir"/not-cherry-picks)" = ""
+		if test -f "$rewritten"/$rev &&
+		   ! sane_grep "$rev" "$state_dir"/not-cherry-picks >/dev/null
 		then
 			# Use -f2 because if rev-list is telling us this commit is
 			# not worthwhile, we don't want to track its multiple heads,