builtin-remote: fix two inconsistencies in the output of "show <remote>"

Remote and stale branches are emitted in alphabetical order, but new and
tracked branches are not. So sort the latter to be consistent with the
former. This also lets us use more efficient string_list_has_string()
instead of unsorted_string_list_has_string().

"show <remote>" prunes symrefs, but "show <remote> -n" does not. Fix the
latter to match the former.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5505-remote.sh

@@ -141,8 +141,8 @@ cat > test/expect << EOF
   New remote branch (next fetch will store in remotes/origin)
     master
   Tracked remote branches
-    side
     master
+    side
   Local branches pushed with 'git push'
     master:upstream
     +refs/tags/lastbackup