git-p4: input to "p4 files" by stdin instead of arguments

This approach, suggested by Alex Riesen, bypasses the need for xargs-style
argument list handling. The handling in question looks broken in a corner
case with SC_ARG_MAX=4096 and final argument over 96 characters.

Signed-off-by: Scott Lamb <slamb@slamb.org>
Signed-off-by: Simon Hausmann <simon@lst.de>