This changes the get_entry() method to return a refcounted version of
the config entry, which you have to free when you're done.

This allows us to avoid freeing the memory in which the entry is stored
on a refresh, which may happen at any time for a live config.

For this reason, get_string() has been forbidden on live configs and a
new function get_string_buf() has been added, which stores the string in
a git_buf which the user then owns.

The functions which parse the string value takea advantage of the
borrowing to parse safely and then release the entry.

Submodule init should handle relative paths in .gitmodules files
and resolve these urls when updating the git config file.

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.