repository: ensure that the path to the .git directory ends with a forward slash when opening a repository through a working directory path

This fixes an issue which was detected while using one of the libgit2 bindings [0]. The lack of the trailing forward slash led the name of references returned by git_reference_listall() to be prefixed with a forward slash.

  [0]: https://github.com/libgit2/libgit2sharp/pull/108