Teach repository to use relative paths for git symbolic links

When cloning submodules, git can place the sub repository inside the parent repository's .git folder, instead of with sub repository's working directory. It links these two through a "git symbolic link". libgit2 already had support for most of this, but it always created the links using absolute paths, where as git creates relative paths.

This change is to allow the creation of repositories with separate the repository and working directories and calculates a the relative path between them.

TODO:
- [ ] Better names for functions?
- [ ] Detect cases when relative paths would not work, and fallback to using absolute paths? (e.g. initializing a repository where the repository and the work dir are on separate drives?)
- [ ] Update Changelog.md
Yeah.  I suppose that `cl_assert`ing from the helper function is ["good Clar style"](https://github.com/vmg/clar).

This is definitely not the way we use it internally, whenever we can avoid it, because `cl_assert` failing on a line number in a helper method is useless.  `cl_assert`ing in a macro that can expand `__LINE__` to the actual line number that's failing is way more useful.

However, we should probably be using "good Clar style" here in libgit2.  :)