Change git_odb__read_packed to return ENOTFOUND until implemented

We didn't search for the object, so we cannot possibly promise it
to the caller of git_odb_read().

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

src/odb.c

@@ -502,6 +502,6 @@ int git_odb__read_loose(git_obj *out, git_odb *db, const git_oid *id)
 
 int git_odb__read_packed(git_obj *out, git_odb *db, const git_oid *id)
 {
-	return GIT_SUCCESS;
+	return GIT_ENOTFOUND;
 }