mktag: allow omitting the header/body \n separator

Change mktag's acceptance rules to accept an empty body without an
empty line after the header again. This fixes an ancient unintended
dregression in "mktag".

When "mktag" was introduced in ec4465adb3 (Add "tag" objects that can
be used to sign other objects., 2005-04-25) the input checks were much
looser. When it was documented it 6cfec03680 (mktag: minimally update
the description., 2007-06-10) it was clearly intended for this \n to
be optional:

    The message, when [it] exists, is separated by a blank line from
    the header.

But then in e0aaf781f6 (mktag.c: improve verification of tagger field
and tests, 2008-03-27) this was made an error, seemingly by
accident. It was just a result of the general header checks, and all
the tests after that patch have a trailing empty line (but did not
before).

Let's allow this again, and tweak the test semantics changed in
e0aaf781f6 to remove the redundant empty line. New tests added in
previous commits of mine already added an explicit test for allowing
the empty line between header and body.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsck.c

@@ -987,6 +987,8 @@ int fsck_tag_standalone(const struct object_id *oid, const char *buffer,
 	}
 	else
 		ret = fsck_ident(&buffer, oid, OBJ_TAG, options);
+	if (!*buffer)
+		goto done;
 
 	if (!starts_with(buffer, "\n")) {
 		/*

t/t3800-mktag.sh

@@ -446,7 +446,7 @@ tagger T A Gger <tagger@example.com> 1206478233 -0500
 
 EOF
 
-test_expect_mktag_success 'require a blank line before an empty body (1)'
+test_expect_mktag_success 'allow a blank line before an empty body (1)'
 
 cat >tag.sig <<EOF
 object $head
@@ -455,8 +455,7 @@ tag mytag
 tagger T A Gger <tagger@example.com> 1206478233 -0500
 EOF
 
-check_verify_failure 'require a blank line before an empty body (2)' \
-	'^error:.* extraHeaderEntry:'
+test_expect_mktag_success 'allow no blank line before an empty body (2)'
 
 ############################################################
 # 24. create valid tag
@@ -466,7 +465,6 @@ object $head
 type commit
 tag mytag
 tagger T A Gger <tagger@example.com> 1206478233 -0500
-
 EOF
 
 test_expect_mktag_success 'create valid tag object'