Fix gclient_scm_test that was broken in r61459.
Fix gclient_scm_test that was broken in r61459.

Added a unit test to verify .hg detection.
Also refactor the test a bit to reduce the amount of reduncdant code.

TEST=fixed unit tests
BUG=none

Review URL: http://codereview.chromium.org/3516015

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@61551 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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