When opening a repository let libgit2 find the appropriate git directory

This is required when the repository's submodules
have been initialised with git 1.8, and their .git
is nested within the root project's.

If opened directly through the git directory,
libgit2 will lose track of which working directory
a user is trying to manipulate; will be unable to
find refs, and other such fun.
GTRepository: remove more redundant error checking in initWithURL:error:

libgit2 will already fail if there's no .git
directory for the path given, and this allows us
to remove the internal `_gitURLForURL:` function
entirely.

libgit2/objective-git#127
Merge pull request #127 from rowanj/fixSubmodulesInGitDir

When opening a repository let libgit2 find the appropriate git directory