commit: support a root commits

A root commit is a commit whose branch (usually what HEAD points to)
doesn't exist (yet). This situation can happen when the commit is the
first after 1) a repository is initialized or 2) a orphan checkout has
been performed.

Take this opportunity to remove the symbolic link check, as
git_reference_resolve works on OID refs as well.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>

src/commit.c

@@ -224,9 +224,18 @@ int git_commit_create(
 		if (error < GIT_SUCCESS)
 			return error;
 
-		if (git_reference_type(head) == GIT_REF_SYMBOLIC) {
-			if ((error = git_reference_resolve(&head, head)) < GIT_SUCCESS)
+		error = git_reference_resolve(&head, head);
+		if (error < GIT_SUCCESS) {
+			if (error != GIT_ENOTFOUND)
 				return error;
+		/*
+		 * The target of the reference was not found. This can happen
+		 * just after a repository has been initialized (the master
+		 * branch doesn't exist yet, as it doesn't have anything to
+		 * point to) or after an orphan checkout, so if the target
+		 * branch doesn't exist yet, create it and return.
+		 */
+			return git_reference_create_oid_f(&head, repo, git_reference_target(head), oid);
 		}
 
 		error = git_reference_set_oid(head, oid);