1166703 - builder.py now checks to make sure master isn't checked out.

Building master is still allowed if the --scratch flag is used, or if
you use the new --build-master flag.

https://bugzilla.redhat.com/show_bug.cgi?id=1166703
can you change this to `/usr/bin/git` ? Some people alias their `git` command to wrapper scripts. I don't know if the call to `check_output()` will do the right thing in this case, it may but might not.