files-backend: use `die("BUG: ...")`, not `die("internal error: ...")`

The former is by far more common in our codebase.
Signed-off-by: NMichael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

refs/files-backend.c

@@ -105,7 +105,7 @@ static void clear_packed_ref_cache(struct files_ref_store *refs)
 		struct packed_ref_cache *packed_refs = refs->packed;
 
 		if (packed_refs->lock)
-			die("internal error: packed-ref cache cleared while locked");
+			die("BUG: packed-ref cache cleared while locked");
 		refs->packed = NULL;
 		release_packed_ref_cache(packed_refs);
 	}
@@ -397,7 +397,7 @@ static void add_packed_ref(struct files_ref_store *refs,
 	struct packed_ref_cache *packed_ref_cache = get_packed_ref_cache(refs);
 
 	if (!packed_ref_cache->lock)
-		die("internal error: packed refs not locked");
+		die("BUG: packed refs not locked");
 	add_ref_entry(get_packed_ref_dir(packed_ref_cache),
 		      create_ref_entry(refname, oid, REF_ISPACKED, 1));
 }
@@ -1324,7 +1324,7 @@ static int commit_packed_refs(struct files_ref_store *refs)
 	files_assert_main_repository(refs, "commit_packed_refs");
 
 	if (!packed_ref_cache->lock)
-		die("internal error: packed-refs not locked");
+		die("BUG: packed-refs not locked");
 
 	out = fdopen_lock_file(packed_ref_cache->lock, "w");
 	if (!out)
@@ -1367,7 +1367,7 @@ static void rollback_packed_refs(struct files_ref_store *refs)
 	files_assert_main_repository(refs, "rollback_packed_refs");
 
 	if (!packed_ref_cache->lock)
-		die("internal error: packed-refs not locked");
+		die("BUG: packed-refs not locked");
 	rollback_lock_file(packed_ref_cache->lock);
 	packed_ref_cache->lock = NULL;
 	release_packed_ref_cache(packed_ref_cache);