Support gitdir: refs in BaseRepositoryBuilder.findGitDir

This allows findGitDir to be used for repositories containing
a .git file with a gitdir: ref to the repository's directory
such as submodule repositories that point to a folder under the
parent repository's .git/modules folder

Change-Id: I2f1ec7215a2208aa90511c065cadc7e816522f62
Signed-off-by: Chris Aniszczyk <zx@twitter.com>