refs: don't clear oid before read_raw_ref in the debug ref backend

The debug_read_raw_ref function clears the oid to null_oid before
calling read_raw_ref, which causes the oid to be lost even when
read_raw_ref successfully reads the reference. This leads to failures
when executing commands like "git branch new_branch <commit_id>" with
GIT_TRACE_REFS=1, as the command cannot find a valid branch point
because the oid is null.

Signed-off-by: Xinyu Ruan <r200981113@gmail.com>

Author: RuanXinyu

refs/debug.c

@@ -227,7 +227,6 @@ static int debug_read_raw_ref(struct ref_store *ref_store, const char *refname,
 	struct debug_ref_store *drefs = (struct debug_ref_store *)ref_store;
 	int res = 0;
 
-	oidcpy(oid, null_oid(ref_store->repo->hash_algo));
 	res = drefs->refs->be->read_raw_ref(drefs->refs, refname, oid, referent,
 					    type, failure_errno);