combine-diff: Fix path quoting

combine-diff: Fix path quoting

Earlier when showing combined diff, the filenames on the ---/+++
header lines were quoted incorrectly.  a/ (or b/) prefix was
output literally and then the path was output, with c-quoting.

This fixes the quoting logic, and while at it, adjusts the code
to use the customizable prefix (a_prefix and b_prefix)
introduced recently.

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