ui-refs.c: Remove global header variable

print_tag_header() is only called from cgit_print_tags() -- the
conditional invocation in print_tag() is never executed since
print_tag() is only called by cgit_print_tags() which already executes
print_tag_header() before (resulting in the global variable being always
set in when the condition is evaluated).

Remove the global variable and the conditional invocation.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>
ui-refs.c: Refactor print_tag()

The code snippets for OBJ_TAG and other object types are almost
equivalent. Merge them and use a couple of inline if conditions to
select proper fields.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>