am -3: allow nonstandard -p<num> option

am -3: allow nonstandard -p<num> option

When falling back to 3-way merge, we run "git apply" to synthesize the
fake ancestor tree by parsing the incoming patch, and another "git apply"
to apply the patch to the fake ancestor tree.  Both invocation need to
be aware of the custom -p<num> setting to parse patches that were prepared
with non-standard src/dst prefix.

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