git-p4 fails when cloning a p4 depo.

git-p4 fails when cloning a p4 depo.

A perforce command with all the files in the repo is generated to get
all the file content.
Here is a patch to break it into multiple successive perforce command
who uses 4K of parameter max, and collect the output for later.

It works, but not for big depos, because the whole perforce depo
content is stored in memory in P4Sync.run(), and it looks like mine is
bigger than 2 Gigs, so I had to kill the process.

[Simon: I added the bit about using SC_ARG_MAX, as suggested by Han-Wen]

Signed-off-by: Benjamin Sergeant <bsergean@gmail.com>
Signed-off-by: Simon Hausmann <simon@lst.de>