object-file: hide internals when we need to reprepare loose sources

There are two different situations where we have to clear the cache of
loose objects:

  - When freeing the loose object source itself to avoid memory leaks.

  - When repreparing the loose object source so that any potentially-
    stale data is getting evicted from the cache.

The former is already handled by `odb_source_loose_free()`. But the
latter case is still done manually by in `odb_reprepare()`, so we are
leaking internals into that code.

Introduce a new `odb_source_loose_reprepare()` function as an equivalent
to `packfile_store_prepare()` to hide these implementation details.
Furthermore, while at it, rename the function `odb_clear_loose_cache()`
to `odb_source_loose_clear()`.

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

odb.c

@@ -1071,7 +1071,7 @@ void odb_reprepare(struct object_database *o)
 	odb_prepare_alternates(o);
 
 	for (source = o->sources; source; source = source->next)
-		odb_clear_loose_cache(source);
+		odb_source_loose_reprepare(source);
 
 	o->approximate_object_count_valid = 0;