sync: Limit -j to file descriptors

Each worker thread requires at least 3 file descriptors to run the
forked 'git fetch' child to operate against the local repository.
Mac OS X has the RLIMIT_NOFILE set to 256 by default, which means
a sync -j128 often fails when the workers run out of pipes within
the Python parent process.

Change-Id: I2cdb14621b899424b079daf7969bc8c16b85b903
Signed-off-by: Shawn O. Pearce <sop@google.com>