Make 'tag.c' use 'ref-filter' APIs for iterating through refs, sorting
and printing of refs. This removes most of the code used in 'tag.c'
replacing it with calls to the 'ref-filter' library.

Make 'tag.c' use the 'filter_refs()' function provided by 'ref-filter'
to filter out tags based on the options set.

For printing tags we use 'show_ref_array_item()' function provided by
'ref-filter'.

We improve the sorting option provided by 'tag.c' by using the sorting
options provided by 'ref-filter'. This causes the test 'invalid sort
parameter on command line' in t7004 to fail, as 'ref-filter' throws an
error for all sorting fields which are incorrect. The test is changed
to reflect the same.

Modify documentation for the same.
Mentored-by: NChristian Couder <christian.couder@gmail.com>
Mentored-by: NMatthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: NKarthik Nayak <karthik.188@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>