branch: sort detached HEAD based on a flag

Change the ref-filter sorting of detached HEAD to check the
FILTER_REFS_DETACHED_HEAD flag, instead of relying on the ref
description filled-in by get_head_description() to start with "(",
which in turn we expect to ASCII-sort before any other reference.

For context, we'd like the detached line to appear first at the start
of "git branch -l", e.g.:

    $ git branch -l
    * (HEAD detached at <hash>)
      master

This doesn't change that, but improves on a fix made in
28438e84e0 (ref-filter: sort detached HEAD lines firstly, 2019-06-18)
and gives the Chinese translation the ability to use its preferred
punctuation marks again.

In Chinese the fullwidth versions of punctuation like "()" are
typically written as (U+FF08 fullwidth left parenthesis), (U+FF09
fullwidth right parenthesis) instead[1]. This form is used in both
po/zh_{CN,TW}.po in most cases where "()" is translated in a string.

Aside from that improvement to the Chinese translation, it also just
makes for cleaner code that we mark any special cases in the ref_array
we're sorting with flags and make the sort function aware of them,
instead of piggy-backing on the general-case of strcmp() doing the
right thing.

As seen in the amended tests this made reverse sorting a bit more
consistent. Before this we'd sometimes sort this message in the
middle, now it's consistently at the beginning or end, depending on
whether we're doing a normal or reverse sort. Having it at the end
doesn't make much sense either, but at least it behaves consistently
now. A follow-up commit will make this behavior under reverse sorting
even better.

I'm removing the "TRANSLATORS" comments that were in the old code
while I'm at it. Those were added in d4919bb288 (ref-filter: move
get_head_description() from branch.c, 2017-01-10). I think it's
obvious from context, string and translation memory in typical
translation tools that these are the same or similar string.

1. https://en.wikipedia.org/wiki/Chinese_punctuation#Marks_similar_to_European_punctuation

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

wt-status.h

@@ -77,8 +77,6 @@ enum wt_status_format {
 	STATUS_FORMAT_UNSPECIFIED
 };
 
-#define HEAD_DETACHED_AT _("HEAD detached at ")
-#define HEAD_DETACHED_FROM _("HEAD detached from ")
 #define SPARSE_CHECKOUT_DISABLED -1
 
 struct wt_status_state {