Only call git checkout when it is not head of the working copy

[stephane-caron]

Closes #115

[coveralls]

Pull Request Test Coverage Report for Build 11896209659

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.423%

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Totals
Change from base Build 11746909263:	0.0%
Covered Lines:	945
Relevant Lines:	970

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💛 - Coveralls

[stephane-caron]

Confirming that, with this change, I'm getting:

$ parallel -i -j 8 python -c 'from robot_descriptions import fr3_mj_description; print({}, fr3_mj_description.MJCF_PATH)' -- 1 2 3 4 5 6 7 8
2 ~/.cache/robot_descriptions/mujoco_menagerie/franka_fr3/fr3.xml
3 ~/.cache/robot_descriptions/mujoco_menagerie/franka_fr3/fr3.xml
1 ~/.cache/robot_descriptions/mujoco_menagerie/franka_fr3/fr3.xml
5 ~/.cache/robot_descriptions/mujoco_menagerie/franka_fr3/fr3.xml
6 ~/.cache/robot_descriptions/mujoco_menagerie/franka_fr3/fr3.xml
4 ~/.cache/robot_descriptions/mujoco_menagerie/franka_fr3/fr3.xml
7 ~/.cache/robot_descriptions/mujoco_menagerie/franka_fr3/fr3.xml
8 ~/.cache/robot_descriptions/mujoco_menagerie/franka_fr3/fr3.xml