ref-filter.c: really don't sort when using --no-sort

ref-filter.c: really don't sort when using --no-sort

When '--no-sort' is passed to 'for-each-ref', 'tag', and 'branch', the
printed refs are still sorted by ascending refname. Change the handling of
sort options in these commands so that '--no-sort' to truly disables
sorting.

'--no-sort' does not disable sorting in these commands is because their
option parsing does not distinguish between "the absence of '--sort'"
(and/or values for tag.sort & branch.sort) and '--no-sort'. Both result in
an empty 'sorting_options' string list, which is parsed by
'ref_sorting_options()' to create the 'struct ref_sorting *' for the
command. If the string list is empty, 'ref_sorting_options()' interprets
that as "the absence of '--sort'" and returns the default ref sorting
structure (equivalent to "refname" sort).

To handle '--no-sort' properly while preserving the "refname" sort in the
"absence of --sort'" case, first explicitly add "refname" to the string list
*before* parsing options. This alone doesn't actually change any behavior,
since 'compare_refs()' already falls back on comparing refnames if two refs
are equal w.r.t all other sort keys.

Now that the string list is populated by default, '--no-sort' is the only
way to empty the 'sorting_options' string list. Update
'ref_sorting_options()' to return a NULL 'struct ref_sorting *' if the
string list is empty, and add a condition to 'ref_array_sort()' to skip the
sort altogether if the sort structure is NULL. Note that other functions
using 'struct ref_sorting *' do not need any changes because they already
ignore NULL values.

Finally, remove the condition around sorting in 'ls-remote', since it's no
longer necessary. Unlike 'for-each-ref' et. al., it does *not* do any
sorting by default. This default is preserved by simply leaving its sort key
string list empty before parsing options; if no additional sort keys are
set, 'struct ref_sorting *' is NULL and sorting is skipped.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>