multiple git procs w/lexically bound magit-process

Change magit-run* so that asynchronous git commands bind the
magit-process variable using let instead of setq.  This allows magit to
run multiple concurrent git processes.

Despite a number of indented lines, this patch really only changes the
single line where the setq is replaced with a let (and of course an
added parenthesis ending the body of the let).