reflog: rename unreachable

In C23, "unreachable" is a macro that invokes undefined behavior if it
is invoked.  To make sure that our code compiles on a variety of C
versions, rename unreachable to "is_unreachable".

Signed-off-by: brian m. carlson <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: bk2204

reflog.c

@@ -210,7 +210,7 @@ static void mark_reachable(struct expire_reflog_policy_cb *cb)
 	cb->mark_list = leftover;
 }
 
-static int unreachable(struct expire_reflog_policy_cb *cb, struct commit *commit, struct object_id *oid)
+static int is_unreachable(struct expire_reflog_policy_cb *cb, struct commit *commit, struct object_id *oid)
 {
 	/*
 	 * We may or may not have the commit yet - if not, look it
@@ -265,7 +265,7 @@ int should_expire_reflog_ent(struct object_id *ooid, struct object_id *noid,
 			return 1;
 		case UE_NORMAL:
 		case UE_HEAD:
-			if (unreachable(cb, old_commit, ooid) || unreachable(cb, new_commit, noid))
+			if (is_unreachable(cb, old_commit, ooid) || is_unreachable(cb, new_commit, noid))
 				return 1;
 			break;
 		}