[vim] Fix argument escaping for Windows batch file

Fix #3620

CHANGELOG.md

@@ -27,6 +27,7 @@ CHANGELOG
     - `change-multi(0)` - disable multi-select mode
 - `become` action is now supported on Windows
     - Unlike in *nix, this does not use `execve(2)`. Instead it spawns a new process and waits for it to finish, so the exact behavior may differ.
+- Fixed argument escaping for Windows cmd.exe
 - Bug fixes and improvements
 
 0.50.0

plugin/fzf.vim

@@ -83,12 +83,28 @@ else
   endfunction
 endif
 
+let s:cmd_control_chars = ['&', '|', '<', '>', '(', ')', '@', '^', '!']
+
 function! s:shellesc_cmd(arg)
-  let escaped = substitute(a:arg, '[&|<>()@^]', '^&', 'g')
-  let escaped = substitute(escaped, '%', '%%', 'g')
-  let escaped = substitute(escaped, '"', '\\^&', 'g')
-  let escaped = substitute(escaped, '\(\\\+\)\(\\^\)', '\1\1\2', 'g')
-  return '^"'.substitute(escaped, '\(\\\+\)$', '\1\1', '').'^"'
+  let e = '"'
+  let slashes = 0
+  for c in split(a:arg, '\zs')
+    if c ==# '\'
+      let slashes += 1
+    elseif c ==# '"'
+      let e .= repeat('\', slashes + 1)
+      let slashes = 0
+    elseif c ==# '%'
+      let e .= '%'
+    elseif index(s:cmd_control_chars, c) >= 0
+      let e .= '^'
+    else
+      let slashes = 0
+    endif
+    let e .= c
+  endfor
+  let e .= repeat('\', slashes) .'"'
+  return e
 endfunction
 
 function! fzf#shellescape(arg, ...)