fast-import: don't allow to note on empty branch

'reset' command makes fast-import start a branch from scratch. It's name
is kept in lookup table but it's sha1 is null_sha1 (special value).
'notemodify' command can be used to add a note on branch head given it's
name. lookup_branch() is used it that case and it doesn't check for
null_sha1. So fast-import writes a note for null_sha1 object instead of
giving a error.

Add a check to deny adding a note on empty branch and add a corresponding
test.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fast-import.c

@@ -2416,6 +2416,8 @@ static void note_change_n(struct branch *b, unsigned char old_fanout)
 	/* <committish> */
 	s = lookup_branch(p);
 	if (s) {
+		if (is_null_sha1(s->sha1))
+			die("Can't add a note on empty branch.");
 		hashcpy(commit_sha1, s->sha1);
 	} else if (*p == ':') {
 		uintmax_t commit_mark = strtoumax(p + 1, NULL, 10);

t/t9300-fast-import.sh

@@ -1987,6 +1987,23 @@ test_expect_success \
 	'Q: verify second note for second commit' \
 	'git cat-file blob refs/notes/foobar:$commit2 >actual && test_cmp expect actual'
 
+cat >input <<EOF
+reset refs/heads/Q0
+
+commit refs/heads/note-Q0
+committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+data <<COMMIT
+Note for an empty branch.
+COMMIT
+
+N inline refs/heads/Q0
+data <<NOTE
+some note
+NOTE
+EOF
+test_expect_success \
+	'Q: deny note on empty branch' \
+	'test_must_fail git fast-import <input'
 ###
 ### series R (feature and option)
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