Introduce git_path_make_relative
Introduce option to use relative paths for repository work directory

Teach git_repository_init_ext to use relative paths for the gitlink
to the work directory. This is used when creating a sub repository
where the sub repository resides in the parent repository's
.git directory.
Merge pull request #2530 from libgit2/jamill/relative_gitlink

Teach repository to use relative paths for git symbolic links