Make :FZF work in Vim from Git Bash (#3798)

* make :FZF work in Vim from Git Bash

Despite its title 'Calling fzf#run with a list as source fail (n)vim is used from git bash' the issue in 

#3777

of running `:FZF` in Vim in Git Bash was apparently only fixed for Neovim in Git Bash on Windows 11, but not for Vim from Git Bash.

In view of this, replacing /C by ///C might be considered a universal fix.

This PR just proposes the patch in #1983 that still seems open.

In view of the fourth item in the most recent 2.45.0 https://github.com/git-for-windows/build-extra/blob/main/ReleaseNotes.md#known-issues little seems to have changed regarding path conversion of arguments containing forward slashes

* prefer doubling slashed instead of generic env. var

If MSYS_NO_PATHCONV=1 is used, then all arguments are preserved, in particular possibly paths passed in s:command.
Therefore, only avoid converting `/C` from `cmd` to a path.

plugin/fzf.vim

@@ -711,7 +711,7 @@ function! s:execute(dict, command, use_height, temps) abort
   elseif has('win32unix') && $TERM !=# 'cygwin'
     let shellscript = s:fzf_tempname()
     call s:writefile([command], shellscript)
-    let command = 'cmd.exe /C '.fzf#shellescape('set "TERM=" & start /WAIT sh -c '.shellscript)
+    let command = 'cmd.exe //C '.fzf#shellescape('set "TERM=" & start /WAIT sh -c '.shellscript)
     let a:temps.shellscript = shellscript
   endif
   if a:use_height