Consistent Tags/BTags with query

The `BTags` command is declared with optional `[QUERY]` while the `Tags`
command is declared with optional `[PREFIX]`.  The query behavior in
`BTags` source generator is less restrictive compared to `Tags` whose
source is generated using the `readtags` external command, where tags
are only listed if `PREFIX` is an exact match.  This commit relaxes the
exact prefix match when `readtags` is used to allow for a fuzzy prefix
similar to the source generator of `BTags`.  The `fzf` fuzzy score
ranking in the `s:tags_sink` will still rank the best fuzzy match on
top, same as if the match was exact like in the current implementation.

The changes in this commit are useful for tags that contain some scope
prefixes like the functions `s:tags_sink` or `fzf#vim#tags` in
vim-script code, in combination with a key mapping of the form

```vim
nnoremap <silent> <leader>l :execute "Tags '" . expand('<cword>')<CR>
nnoremap <silent> <leader>bl :execute "BTags '" . expand('<cword>')<CR>
```

If the cursor is placed on the word "tags" in this example
`fzf#vim#tags`, the relaxed `Tags` query call will list the tags entry
if `<cword>` expands to "tags", while the current implementation will
not.  Inconsistently, the `BTags` call will list the tags entry with its
current implementation.

README.md

@@ -71,7 +71,7 @@ so you can omit it if you use a plugin manager that doesn't support hooks.
 - `Ag` requires [The Silver Searcher (ag)][ag]
 - `Rg` requires [ripgrep (rg)][rg]
 - `Tags` and `Helptags` require Perl
-- `Tags PREFIX` requires `readtags` command from [Universal Ctags](https://ctags.io/)
+- `Tags QUERY` requires `readtags` command from [Universal Ctags](https://ctags.io/)
 
 ```sh
 # Installing dependencies using Homebrew
@@ -93,7 +93,7 @@ Commands
 | `:RG [PATTERN]`        | [rg][rg] search result; relaunch ripgrep on every keystroke                           |
 | `:Lines [QUERY]`       | Lines in loaded buffers                                                               |
 | `:BLines [QUERY]`      | Lines in the current buffer                                                           |
-| `:Tags [PREFIX]`       | Tags in the project (`ctags -R`)                                                      |
+| `:Tags [QUERY]`        | Tags in the project (`ctags -R`)                                                      |
 | `:BTags [QUERY]`       | Tags in the current buffer                                                            |
 | `:Changes`             | Changelist across all open buffers                                                    |
 | `:Marks`               | Marks                                                                                 |

autoload/fzf/vim.vim

@@ -1148,7 +1148,7 @@ function! fzf#vim#tags(query, ...)
     return s:warn('Tags command requires perl')
   endif
   if len(a:query) && !executable('readtags')
-    return s:warn('readtags from universal-ctags is required to pre-filter tags with a prefix')
+    return s:warn('readtags from universal-ctags is required to pre-filter tags with a query')
   endif
 
   if empty(tagfiles())

bin/tags.pl

@@ -2,15 +2,15 @@
 
 use strict;
 
-my $prefix = shift @ARGV;
+my $query = shift @ARGV;
 
 foreach my $file (@ARGV) {
   my $lines;
-  if ($prefix eq "") {
+  if ($query eq "") {
     open $lines, $file;
   } else {
     # https://perldoc.perl.org/perlopentut#Expressing-the-command-as-a-list
-    open $lines, '-|', 'readtags', '-t', $file, '-e', '-p', '-', $prefix;
+    open $lines, '-|', 'readtags', '-t', $file, '-e', '-p', '-Q', "(#/^[^[:space:]]*$query\$/ \$name)", '-l';
   }
   while (<$lines>) {
     unless (/^\!/) {