branch.c: use 'ref-filter' APIs

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

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

We provide a sorting option provided for 'branch.c' by using the
sorting options provided by 'ref-filter'. Also by default, we sort by
'refname'.  Since 'HEAD' is alphabatically before 'refs/...' we end up
with an array consisting of the 'HEAD' ref then the local branches and
finally the remote-tracking branches.

Also remove the 'ignore' variable from ref_array_item as it was
previously used for the '--merged' option and now that is handled by
ref-filter.

Modify some of the tests in t1430 to check the stderr for a warning
regarding the broken ref. This is done as ref-filter throws a warning
for broken refs rather than directly printing them.

Add tests and documentation for the same.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.h

@@ -36,7 +36,6 @@ struct ref_array_item {
 	unsigned char objectname[20];
 	int flag;
 	unsigned int kind;
-	int ignore : 1; /* To be removed in the next patch */
 	const char *symref;
 	struct commit *commit;
 	struct atom_value *value;