mktag tests: remove needless SHA-1 hardcoding

Change the tests amended in acb49d1cc8 (t3800: make hash-size
independent, 2019-08-18) even more to make them independent of either
SHA-1 or SHA-256.

Some of these tests were failing for the wrong reasons. The first one
being modified here would fail because the line starts with "xxxxxx"
instead of "object", the rest of the line doesn't matter.

Let's just put a valid hash on the rest of the line anyway to narrow
the test down for just the s/object/xxxxxx/ case.

The second one being modified here would fail under
GIT_TEST_DEFAULT_HASH=sha256 because <some sha-1 length garbage> is an
invalid SHA-256, but we should really be testing <some sha-256 length
garbage> when under SHA-256.

This doesn't really matter since we should be able to trust other
parts of the code to validate things in the 0-9a-f range, but let's
keep it for good measure.

There's a later test which tests an invalid SHA which looks like a
valid one, to stress the "We refuse to tag something we can't
verify[...]" logic in mktag.c.

But here we're testing for a SHA-length string which contains
characters outside of the /[0-9a-f]/i set.

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

t/t3800-mktag.sh

@@ -41,7 +41,7 @@ check_verify_failure 'Tag object length check' \
 #  2. object line label check
 
 cat >tag.sig <<EOF
-xxxxxx 139e9b33986b1c2670fff52c5067603117b3e895
+xxxxxx $head
 type tag
 tag mytag
 tagger . <> 0 +0000
@@ -51,10 +51,10 @@ EOF
 check_verify_failure '"object" line label check' '^error: char0: .*"object "$'
 
 ############################################################
-#  3. object line SHA1 check
+#  3. object line hash check
 
 cat >tag.sig <<EOF
-object zz9e9b33986b1c2670fff52c5067603117b3e895
+object $(echo $head | tr 0-9a-f z)
 type tag
 tag mytag
 tagger . <> 0 +0000