Fixup missed parameter rename.

https://rbcommons.com/s/twitter/r/871/ comitted @ 50bdab06 failed CI here: https://travis-ci.org/pantsbuild/pants/jobs/32402628
This fixes that.

Testing Done:
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test/scm/::
...
23:16:08 00:00   [test]
23:16:08 00:00     [pytest]
23:16:08 00:00       [run]
                     ============== test session starts ===============
                     platform linux2 -- Python 2.6.9 -- py-1.4.23 -- pytest-2.6.1
                     plugins: cov, timeout
                     collected 4 items

                     tests/python/pants_test/scm/test_git.py ....

                     ============ 4 passed in 0.60 seconds ============

23:16:28 00:20     [junit]
23:16:28 00:20     [specs]
               SUCCESS

Bugs closed: 476

Reviewed at https://rbcommons.com/s/twitter/r/875/