Fix Support for PowerShell under Windows with VIM 8+

[tiamat18]

The following 'vim-plug' commands would fail and/or display errors when executed on by VIM 8+ on a Windows platform with the 'shell' option set to 'powershell' or 'pwsh':

• PlugInstall
• PlugUpdate
• PlugClean
• PlugStatus
• PlugDiff

There were two causes for these errors:

• A bug in VIM itself (resolved by patch 9.2.6) with how compound PowerShell commands are handled by the 'system()' vimscript function.
• A bug in the 's:vim8' branch of the private 'vim-plug' function 's:spawn()' where the actual command to be executed was escaped for 'cmd.exe' but the 'cd' prefix added by 's:with_cd()" was escaped for PowerShell.

Describe the details of your PR ...

[HaaiHenkie]

Nice fix — making the shell explicit in s:with_cd is definitely the right approach. It solved all my vim 8+ problems with PowerShell under Windows. I hope it will be implemented soon.

For completeness and consistency, consider passing {'shell': 'cmd.exe'} to s:with_cd() in s:bang() as well, though I don't think it is really necessary. This is because I don't know whether this function is ever used under Windows and, whether it will work anyway, taking into account the FIXME remark. Since in s:bang() the command is executed via a batch file under Windows, the command shell will be cmd.exe even when executed under PowerShell. So to prevent PowerShell type commands in a batch file, I would suggest the following change on line 1000:

let cmd = a:0 ? s:with_cd(a:cmd, a:1, {'shell': s:is_win ? 'cmd.exe' : &shell}) : a:cmd