Ignore git errors on checkout if checkout was successful.

In rare cases, git will exit with an error even though it was able to check out the commit.

For example, if a file has type symlink but invalid content:
http://stackoverflow.com/questions/18411200/git-unable-to-create-symlink-file-name-too-long

In this case, git will exit with status 1 and with a message like:

    error: unable to create symlink cms/static/sass/_mixins.scss (File name too long)

To handle this case, we check the current commit hash and compare it to the hash we tried to check out.  If they are equal (meaning the commit was successfully checked out), then suppress the `GitException`.