Don't complain if git isn't installed

Currently, if git isn't installed you get "sh: git: command not found\n" printed to stderr.  A better solution might be to search for git in the PATH and only run it if it is found, but I don't see a reason to reimplement the search sh can already do.  Another option would be to have a --with-git= in the configure script (people without git could pass --with-git=/bin/true), but that would require a recompile if git were to be installed later.  A third, and possibly best, option would be to have a config file where you could specify which executable to run (if at all), but again, that is more code.  I also don't see a need for any error messages git might produce, so discarding stderr for this command is probably the right answer anyway.  If it fails, ag will just search more than intended.
Merge pull request #340 from cowens/patch-1

Don't complain if git isn't installed