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>

1 files changed, 6 insertions, 1 deletions

diff --git a/builtin/fast-export.c b/builtin/fast-export.c
index f541f55d333..5822271c6b0 100644
--- a/builtin/fast-export.c
+++ b/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",