[vim] Don't pipe FZF_DEFAULT_COMMAND in Windows

[janlazo]

cmd.exe leaves the pipe hanging for the source command after fzf finishes so powershell and ag keep running and cmd.exe does not close. Putting FZF_DEFAULT_COMMAND in a batchfile hides the issue from cmd.exe so Vim is stuck waiting and Neovim does not run the exit callback. Letting fzf run FZF_DEFAULT_COMMAND passes the responsibility of stopping the source program to fzf executable so Vim/Neovim don't have to wait for the source command to finish executing.

This issue does not happen when running the fzf executable directly in the shell if FZF_DEFAULT_COMMAND is using powershell. ag still gets stuck.

I did not consider ack because it requires perl and is too slow as a default command.

[janlazo]

The external cmd.exe window does not close for gvim and neovim-qt (or any Neovim GUI) because the source command is still running. Running powershell or ag consecutively on large directories slows down the system if they're not killed so they're not viable yet for default commands.

[junegunn]

I see, it was introduced to avoid possible slowness of $SHELL (#552), which should be a non-issue on Windows platform.

[junegunn]

Merged, thanks!