odb: Add `git_odb_add_disk_alternate`

Loads a disk alternate by path to the ODB. Mimics the
`GIT_ALTERNATE_OBJECT_DIRECTORIES` shell var.

include/git2/odb.h

@@ -89,6 +89,23 @@ GIT_EXTERN(int) git_odb_add_backend(git_odb *odb, git_odb_backend *backend, int
 GIT_EXTERN(int) git_odb_add_alternate(git_odb *odb, git_odb_backend *backend, int priority);
 
 /**
+ * Add an on-disk alternate to an existing Object DB.
+ *
+ * Note that the added path must point to an `objects`, not
+ * to a full repository, to use it as an alternate store.
+ *
+ * Alternate backends are always checked for objects *after*
+ * all the main backends have been exhausted.
+ *
+ * Writing is disabled on alternate backends.
+ *
+ * @param odb database to add the backend to
+ * @param path path to the objects folder for the alternate
+ * @return 0 on success; error code otherwise
+ */
+GIT_EXTERN(int) git_odb_add_disk_alternate(git_odb *odb, const char *path);
+
+/**
  * Close an open object database.
  *
  * @param db database pointer to close. If NULL no action is taken.

src/odb.c

@@ -470,6 +470,11 @@ static int load_alternates(git_odb *odb, const char *objects_dir, int alternate_
 	return result;
 }
 
+int git_odb_add_disk_alternate(git_odb *odb, const char *path)
+{
+	return add_default_backends(odb, path, 1, 0);
+}
+
 int git_odb_open(git_odb **out, const char *objects_dir)
 {
 	git_odb *db;
@@ -481,8 +486,7 @@ int git_odb_open(git_odb **out, const char *objects_dir)
 	if (git_odb_new(&db) < 0)
 		return -1;
 
-	if (add_default_backends(db, objects_dir, 0, 0) < 0)
-	{
+	if (add_default_backends(db, objects_dir, 0, 0) < 0) {
 		git_odb_free(db);
 		return -1;
 	}