Canonicalize the two fzf key aliases before comparing

fzf reads 'return' as 'enter' and 'alt-return' as 'alt-enter', so
show_key='alt-return' passed the check against the default paste key and
--expect closed fzf while the footer advertised Show. ctrl-m is not an alias,
it goes through the generic ctrl- branch to its own event.

autoload/fzf/vim.vim

@@ -361,7 +361,7 @@ function! s:build_hint(entries, defaults, ...)
   let keys = []
   let actions = get(g:, 'fzf_action', s:default_action)
   if a:defaults
-    if index(taken, 'enter') < 0 && index(taken, 'return') < 0
+    if index(taken, 'enter') < 0
       call add(entries, ['Enter', 'Open'])
     endif
     for [key, name, label] in [['ctrl-x', 'C-X', 'HSplit'], ['ctrl-v', 'C-V', 'VSplit'], ['ctrl-t', 'C-T', 'Tab']]
@@ -442,6 +442,9 @@ function! s:show_key()
   return s:conf('show_key', 'ctrl-o')
 endfunction
 
+" fzf spells these two events more than one way
+let s:key_aliases = {'return': 'enter', 'alt-return': 'alt-enter'}
+
 " A key can be a comma-separated list, which fzf binds to every key in it.
 " fzf escapes 'alt-,' before splitting on the comma, so do the same. It also
 " lowercases a key name before matching it, except for 'alt-X' and a bare
@@ -452,8 +455,9 @@ function! s:split_keys(keys)
   let keys = map(split(substitute(a:keys, '\c\(alt-\),', '\1'.sep, 'g'), ','),
         \ 'substitute(v:val, sep, ",", "g")')
   " 'alt-X' keeps X as typed but not its prefix, so fold only the prefix there
-  return map(keys, 'strchars(v:val) == 1 ? v:val
+  call map(keys, 'strchars(v:val) == 1 ? v:val
         \ : v:val =~? "^alt-.$" ? "alt-".v:val[4:] : tolower(v:val)')
+  return map(keys, 'get(s:key_aliases, v:val, v:val)')
 endfunction
 
 " 'ctrl-alt-x' -> 'C-A-X' and 'double-click' -> 'Dbl-Click', to match the