t5604: do not expect that HEAD can be a valid tagname

09116a1c (refs: loosen over-strict "format" check, 2011-11-16) introduced a test piece (originally in t5700) that expects to be able to create a tag named "HEAD" and then a local clone using the repository as its own reference works correctly. Later, another test piece started using this tag starting at acede2eb (t5700: document a failure of alternates to affect fetch, 2012-02-11). But the breakage 09116a1c fixed was not specific to the tagname HEAD. It would have failed exactly the same way if the tag used were foo instead of HEAD. Before forbidding "git tag" from creating "refs/tags/HEAD", update these tests to use 'foo', not 'HEAD', as the name of the test tag. Note that the test piece that uses the tag learned the value of the tag in unnecessarily inefficient and convoluted way with for-each-ref. Just use "rev-parse" instead. Signed-off-by: Junio C Hamano <gitster@pobox.com>