Limit `arc branch` to 16 concurrent processes

Summary:
Currently, this spawns 125 concurrent processes on my machine, which overflows some limit and gives me an error:

  PHP Warning:  proc_open(): unable to create pipe Too many open files in /INSECURE/devtools/libphutil/src/future/exec/ExecFuture.php on line 491

Instead, limit parallelism to 16. The runtime is approximately the same for me, and dominated by other concerns (conduit calls).

Test Plan: Ran `arc branch` successfully. Ran `arc branch --trace`, observed behavior.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4336