Loosen ensure_not_bare rules in checkout

With the new target directory option to checkout, the non-bareness
of the repository should be checked much later in the parameter
validation process - actually that check was already in place, but
I was doing it redundantly in the checkout APIs.

This removes the now unnecessary early check for bare repos.  It
also adds some other parameter validation and makes it so that
implied parameters can actually be passed as NULL (i.e. if you
pass a git_index, you don't have to pass the git_repository - we
can get it from index).

src/checkout.c

@@ -1369,8 +1369,19 @@ int git_checkout_index(
 	int error;
 	git_iterator *index_i;
 
-	if ((error = git_repository__ensure_not_bare(repo, "checkout index")) < 0)
-		return error;
+	if (!index && !repo) {
+		giterr_set(GITERR_CHECKOUT,
+			"Must provide either repository or index to checkout");
+		return -1;
+	}
+	if (index && repo && git_index_owner(index) != repo) {
+		giterr_set(GITERR_CHECKOUT,
+			"Index to checkout does not match repository");
+		return -1;
+	}
+
+	if (!repo)
+		repo = git_index_owner(index);
 
 	if (!index && (error = git_repository_index__weakptr(&index, repo)) < 0)
 		return error;
@@ -1394,8 +1405,19 @@ int git_checkout_tree(
 	git_tree *tree = NULL;
 	git_iterator *tree_i = NULL;
 
-	if ((error = git_repository__ensure_not_bare(repo, "checkout tree")) < 0)
-		return error;
+	if (!treeish && !repo) {
+		giterr_set(GITERR_CHECKOUT,
+			"Must provide either repository or tree to checkout");
+		return -1;
+	}
+	if (treeish && repo && git_object_owner(treeish) != repo) {
+		giterr_set(GITERR_CHECKOUT,
+			"Object to checkout does not match repository");
+		return -1;
+	}
+
+	if (!repo)
+		repo = git_object_owner(treeish);
 
 	if (git_object_peel((git_object **)&tree, treeish, GIT_OBJ_TREE) < 0) {
 		giterr_set(
@@ -1420,8 +1442,7 @@ int git_checkout_head(
 	git_tree *head = NULL;
 	git_iterator *head_i = NULL;
 
-	if ((error = git_repository__ensure_not_bare(repo, "checkout head")) < 0)
-		return error;
+	assert(repo);
 
 	if (!(error = checkout_lookup_head_tree(&head, repo)) &&
 		!(error = git_iterator_for_tree(&head_i, head, 0, NULL, NULL)))