odb_read_prefix: reset error in backends loop

When looking for an object by prefix, we query all the backends so that
we can ensure that there is no ambiguity.  We need to reset the `error`
value between backends; otherwise the first backend may find an object
by prefix, but subsequent backends may not.  If we do not reset the
`error` value then it will remain at `GIT_ENOTFOUND` and `read_prefix_1`
will fail, despite having actually found an object.

src/odb.c

@@ -1116,8 +1116,11 @@ static int read_prefix_1(git_odb_object **out, git_odb *db,
 		if (b->read_prefix != NULL) {
 			git_oid full_oid;
 			error = b->read_prefix(&full_oid, &raw.data, &raw.len, &raw.type, b, key, len);
-			if (error == GIT_ENOTFOUND || error == GIT_PASSTHROUGH)
+
+			if (error == GIT_ENOTFOUND || error == GIT_PASSTHROUGH) {
+				error = 0;
 				continue;
+			}
 
 			if (error)
 				goto out;