[Tags] Use 'readtags' if query string is given

This pre-filtering is critical when working with large tag files.

'readtags' can perform a binary search for the given prefix string
but fzf has to load the entire file into memory just to start searching.

Close #1529
Close #1524

autoload/fzf/vim.vim

@@ -1110,8 +1110,9 @@ function! fzf#vim#tags(query, ...)
   endfor
   let opts = v2_limit < 0 ? ['--algo=v1'] : []
 
+  let args = insert(map(tagfiles, 'fzf#shellescape(fnamemodify(v:val, ":p"))'), fzf#shellescape(a:query), 0)
   return s:fzf('tags', {
-  \ 'source':  'perl '.fzf#shellescape(s:bin.tags).' '.join(map(tagfiles, 'fzf#shellescape(fnamemodify(v:val, ":p"))')),
+  \ 'source':  join(['perl', fzf#shellescape(s:bin.tags), join(args)]),
   \ 'sink*':   s:function('s:tags_sink'),
   \ 'options': extend(opts, ['--nth', '1..2', '-m', '-d', '\t', '--tiebreak=begin', '--prompt', 'Tags> ', '--query', a:query])}, a:000)
 endfunction

bin/tags.pl

@@ -2,8 +2,16 @@
 
 use strict;
 
+my $prefix = shift @ARGV;
+
 foreach my $file (@ARGV) {
-  open my $lines, $file;
+  my $lines;
+  if ($prefix eq "") {
+    open $lines, $file;
+  } else {
+    # https://perldoc.perl.org/perlopentut#Expressing-the-command-as-a-list
+    open $lines, '-|', 'readtags', '-t', $file, '-e', '-p', $prefix;
+  }
   while (<$lines>) {
     unless (/^\!/) {
       s/^[^\t]*/sprintf("%-24s", $&)/e;