vim: find wiki backlinks

This code uses some clever filename expansion: we expand to a full path
with :p, then substitute off whatever g:wiki_root is with :s?pat?repl?
and an empty replacement. This will break if your g:wiki_root has a ? or
pattern metacharacters, though :/ Finally, we lop off the leading slash
from absolute paths.

The search will also break if the filename has metacharactes in it for
:grep, which for me is rg at the moment.

links/vim/after/plugin/config/wiki.vim

@@ -44,6 +44,11 @@ call popsikey#register('<Leader>w', [
       \   info:   'edit notepad',
       \   action: ":Ewiki notepad\<CR>",
       \ },
+      \ #{
+      \   key:    'b',
+      \   info:   'find backlinks',
+      \   action: ":execute 'grep' expand('%:p:s?' .. g:wiki_root .. '??')[1:] g:wiki_root\<CR>",
+      \ },
       \ ],
       \ #{
       \   title: ' Wiki ',