Fix regression around escape sequences

We mapped esc to cancel the prompt, but when using Vim (not an issue in Nvim due
to its proper key parsing via libtermkey), even arrow keys would cancel the
prompt because they emit sequences like \eOA, \eOB, etc.

Now we reverse the logic. The default mode is to cancel the prompt unless <cr>
(or the equivalent <c-m>) is typed.

This was once fixed in
mhinz@2271e35
but regressed when we removed the need for magic strings.

Fixes mhinz#221

Author: mhinz

plugin/grepper.vim

@@ -761,13 +761,13 @@ function! s:prompt(flags)
   endif
 
   " Store original mappings
-  let mapping_esc  = maparg('<esc>', 'c', '', 1)
+  let mapping_cr   = maparg('<cr>', 'c', '', 1)
   let mapping_tool = maparg(get(g:grepper, 'next_tool', g:grepper.prompt_mapping_tool), 'c', '', 1)
   let mapping_dir  = maparg(g:grepper.prompt_mapping_dir,  'c', '', 1)
   let mapping_side = maparg(g:grepper.prompt_mapping_side, 'c', '', 1)
 
   " Set plugin-specific mappings
-  cnoremap <silent> <esc> <c-\>e<sid>set_prompt_op('cancelled')<cr><c-c>
+  cnoremap <silent> <cr> <c-\>e<sid>set_prompt_op('cr')<cr><cr>
   execute 'cnoremap <silent>' g:grepper.prompt_mapping_tool "\<c-\>e\<sid>set_prompt_op('flag_tool')<cr><cr>"
   execute 'cnoremap <silent>' g:grepper.prompt_mapping_dir  "\<c-\>e\<sid>set_prompt_op('flag_dir')<cr><cr>"
   execute 'cnoremap <silent>' g:grepper.prompt_mapping_side "\<c-\>e\<sid>set_prompt_op('flag_side')<cr><cr>"
@@ -793,7 +793,7 @@ function! s:prompt(flags)
   "   'flag_dir':   don't start searching; toggle -dir flag
   "   'flag_side':  don't start searching; toggle -side flag
   "   'cr':         start searching
-  let s:prompt_op = 'cr'
+  let s:prompt_op = 'cancelled'
 
   echohl GrepperPrompt
   call inputsave()
@@ -816,11 +816,11 @@ function! s:prompt(flags)
     redraw!
 
     " Restore mappings
-    cunmap <esc>
+    cunmap <cr>
     execute 'cunmap' g:grepper.prompt_mapping_tool
     execute 'cunmap' g:grepper.prompt_mapping_dir
     execute 'cunmap' g:grepper.prompt_mapping_side
-    call s:restore_mapping(mapping_esc)
+    call s:restore_mapping(mapping_cr)
     call s:restore_mapping(mapping_tool)
     call s:restore_mapping(mapping_dir)
     call s:restore_mapping(mapping_side)