odb: introduce odb_source_new()

We have three different locations where we create a new ODB source.
Deduplicate the logic via a new `odb_source_new()` function.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

odb.h

@@ -89,6 +89,10 @@ struct odb_source {
 	char *path;
 };
 
+struct odb_source *odb_source_new(struct object_database *odb,
+				  const char *path,
+				  bool local);
+
 struct packed_git;
 struct packfile_store;
 struct cached_object_entry;