Correct the definition of "empty" in the docs for git_repository_is_empty

This improves the documentation for `git_repository_is_empty`
which currently does not accurately describe libgit2's definition
of "empty".

It says that HEAD must point to the "unborn master branch", when
in fact, this is not the case if the repo's `init.defaultBranch`
configuration is set. If it is set, it will check that HEAD points
there. Only if it is not set does it fall back to `master`.

Author: timrogers

include/git2/repository.h

@@ -465,7 +465,9 @@ GIT_EXTERN(int) git_repository_head_unborn(git_repository *repo);
  * Check if a repository is empty
  *
  * An empty repository has just been initialized and contains no references
- * apart from HEAD, which must be pointing to the unborn master branch.
+ * apart from HEAD, which must be pointing to the unborn master branch,
+ * or the branch specified for the repository in the `init.defaultBranch`
+ * configuration variable.
  *
  * @param repo Repo to test
  * @return 1 if the repository is empty, 0 if it isn't, error code