Fix preview support for MS Windows (WSL and Git bash) (#1215)

* Fixes #1212

* Path translation only when containing backslash.

- lower letters also allowed for drive

Co-authored-by: Jan Stocker <Jan.Stocker@cosmoconsult.com>

autoload/fzf/vim.vim

@@ -163,6 +163,12 @@ function! fzf#vim#with_preview(...)
   endif
   if s:is_win
     let is_wsl_bash = exepath('bash') =~? 'Windows[/\\]system32[/\\]bash.exe$'
+    if empty($MSWINHOME)
+      let $MSWINHOME = $HOME
+    endif
+    if is_wsl_bash && $WSLENV !~# '[:]\?MSWINHOME\(\/[^:]*\)\?\(:\|$\)'
+      let $WSLENV = 'MSWINHOME/u:'.$WSLENV
+    endif
     let preview_cmd = 'bash '.(is_wsl_bash
     \ ? substitute(substitute(s:bin.preview, '^\([A-Z]\):', '/mnt/\L\1', ''), '\', '/', 'g')
     \ : escape(s:bin.preview, '\'))