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>