Only use -c flag when git supports it
Only use -c flag when git supports it

The `-c` flag for git is only supported in git 1.8.0+. We now only use
it when we detect an appropriate git version.

Testing Done:
Ran unit tests.

Verified the Git version was determined correctly.

Bugs closed: 3876

Reviewed at https://reviews.reviewboard.org/r/7337/

davidt/rbtools,haosdent/rbtools,beol/rbtools,davidt/rbtools,datjwu/rbtools,beol/rbtools,beol/rbtools,haosdent/rbtools,reviewboard/rbtools,haosdent/rbtools,reviewboard/rbtools,davidt/rbtools,datjwu/rbtools,reviewboard/rbtools,datjwu/rbtools