help.c::uniq: plug a leak

help.c::uniq: plug a leak

We observe that the j-1 element can serve the same purpose as the i-1
element that we use in the strcmp(); it is either:

  1. Exactly i-1, when the loop begins (and until we see a duplicate).

  2. The same pointer that was stored at i-1 (if it was not a duplicate,
     and we just copied it into place).

  3. A pointer to an equivalent string (i.e., we rejected i-1 _because_
     it was identical to j-1).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>