Support gitdir references in working tree .git file

Support gitdir references in working tree .git file

A '.git' file in a repository's working tree root is now parsed
as a ref to a folder located elsewhere.  This supports submodules
having their repository location outside of the parent repository's
working directory such as in the parent repository's '.git/modules'
directory.

This adds support to BaseRepositoryBuilder for repositories created
with the '--separate-git-dir' option specified to 'git init'.

Change-Id: I73c538f6d845bdbc0c4e2bce5a77f900cf36e1a9
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>