fast-import: allow tags to be identified by mark labels

Mark identifiers are used in fast-export and fast-import to provide a
label to refer to earlier content.  Blobs are given labels because they
need to be referenced in the commits where they first appear with a
given filename, and commits are given labels because they can be the
parents of other commits.  Tags were never given labels, probably
because they were viewed as unnecessary, but that presents two problems:

   1. It leaves us without a way of referring to previous tags if we
      want to create a tag of a tag (or higher nestings).
   2. It leaves us with no way of recording that a tag has already been
      imported when using --export-marks and --import-marks.

Fix these problems by allowing an optional mark label for tags.

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

Documentation/git-fast-import.txt

@@ -774,6 +774,7 @@ lightweight (non-annotated) tags see the `reset` command below.
 
 ....
 	'tag' SP <name> LF
+	mark?
 	'from' SP <commit-ish> LF
 	original-oid?
 	'tagger' (SP <name>)? SP LT <email> GT SP <when> LF

fast-import.c

@@ -2713,6 +2713,7 @@ static void parse_new_tag(const char *arg)
 		first_tag = t;
 	last_tag = t;
 	read_next_command();
+	parse_mark();
 
 	/* from ... */
 	if (!skip_prefix(command_buf.buf, "from ", &from))
@@ -2769,7 +2770,7 @@ static void parse_new_tag(const char *arg)
 	strbuf_addbuf(&new_data, &msg);
 	free(tagger);
 
-	if (store_object(OBJ_TAG, &new_data, NULL, &t->oid, 0))
+	if (store_object(OBJ_TAG, &new_data, NULL, &t->oid, next_mark))
 		t->pack_id = MAX_PACK_ID;
 	else
 		t->pack_id = pack_id;

t/t9300-fast-import.sh

@@ -94,6 +94,23 @@ test_expect_success 'A: create pack from stdin' '
 	reset refs/tags/to-be-deleted
 	from 0000000000000000000000000000000000000000
 
+	tag nested
+	mark :6
+	from :4
+	data <<EOF
+	Tag of our lovely commit
+	EOF
+
+	reset refs/tags/nested
+	from 0000000000000000000000000000000000000000
+
+	tag nested
+	mark :7
+	from :6
+	data <<EOF
+	Tag of tag of our lovely commit
+	EOF
+
 	INPUT_END
 	git fast-import --export-marks=marks.out <input &&
 	git whatchanged master
@@ -176,6 +193,8 @@ test_expect_success 'A: verify marks output' '
 	:3 $(git rev-parse --verify master:file3)
 	:4 $(git rev-parse --verify master:file4)
 	:5 $(git rev-parse --verify master^0)
+	:6 $(git cat-file tag nested | grep object | cut -d" " -f 2)
+	:7 $(git rev-parse --verify nested)
 	EOF
 	test_cmp expect marks.out
 '