Account for optional escaping in tab complete

Tab complete results are always passed to fnameescape(), which can
result in the output having more backslashes than the input, causing
those results to be filtered out.  Attempt to reconcile this.

References: https://github.com/tpope/vim-fugitive/issues/1916

autoload/fugitive.vim

@@ -2328,12 +2328,14 @@ call s:add_methods('buffer', ['repo', 'type'])
 
 function! s:FilterEscape(items, ...) abort
   let items = copy(a:items)
-  call map(items, 's:fnameescape(v:val)')
-  if a:0 && type(a:1) == type('')
-    let cmp = s:FileIgnoreCase(1) ? '==?' : '==#'
-    call filter(items, 'strpart(v:val, 0, strlen(a:1)) ' . cmp . ' a:1')
+  call map(items, 'fnameescape(v:val)')
+  if !a:0 || type(a:1) != type('')
+    let match = ''
+  else
+    let match = substitute(a:1, '^[+>]\|\\\@<![' . substitute(s:fnameescape, '\\', '', '') . ']', '\\&', 'g')
   endif
-  return items
+  let cmp = s:FileIgnoreCase(1) ? '==?' : '==#'
+  return filter(items, 'strpart(v:val, 0, strlen(match)) ' . cmp . ' match')
 endfunction
 
 function! s:GlobComplete(lead, pattern, ...) abort