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

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>