fast-export: fix exporting a tag and nothing else

fast-export allows specifying revision ranges, which can be used to
export a tag without exporting the commit it tags.  fast-export handled
this rather poorly: it would emit a "from :0" directive.  Since marks
start at 1 and increase, this means it refers to an unknown commit and
fast-import will choke on the input.

When we are unable to look up a mark for the object being tagged, use a
"from $HASH" directive instead to fix this problem.

Note that this is quite similar to the behavior fast-export exhibits
with commits and parents when --reference-excluded-parents is passed
along with an excluded commit range.  For tags of excluded commits we do
not require the --reference-excluded-parents flag because we always have
to tag something.  By contrast, when dealing with commits, pruning a
parent is always a viable option, so we need the flag to specify that
parent pruning is not wanted.  (It is slightly weird that
--reference-excluded-parents isn't the default with a separate
--prune-excluded-parents flag, but backward compatibility concerns
resulted in the current defaults.)

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fast-export.c

@@ -860,7 +860,12 @@ static void handle_tag(const char *name, struct tag *tag)
 
 	if (starts_with(name, "refs/tags/"))
 		name += 10;
-	printf("tag %s\nfrom :%d\n", name, tagged_mark);
+	printf("tag %s\n", name);
+	if (tagged_mark)
+		printf("from :%d\n", tagged_mark);
+	else
+		printf("from %s\n", oid_to_hex(&tagged->oid));
+
 	if (show_original_ids)
 		printf("original-oid %s\n", oid_to_hex(&tag->object.oid));
 	printf("%.*s%sdata %d\n%.*s\n",

t/t9350-fast-export.sh

@@ -53,6 +53,19 @@ test_expect_success 'fast-export | fast-import' '
 
 '
 
+test_expect_success 'fast-export ^muss^{commit} muss' '
+	git fast-export --tag-of-filtered-object=rewrite ^muss^{commit} muss >actual &&
+	cat >expected <<-EOF &&
+	tag muss
+	from $(git rev-parse --verify muss^{commit})
+	$(git cat-file tag muss | grep tagger)
+	data 9
+	valentin
+
+	EOF
+	test_cmp expected actual
+'
+
 test_expect_success 'fast-export master~2..master' '
 
 	git fast-export master~2..master >actual &&