git-gui: check whether systems nice command works or disable it
This fixes issue 394 from msysgit. It seems that the Gnuwin32 project provides a nice command but it returns a "not implemented" error. To help users we now try to execute once and disable it in case it fails.
Signed-off-by: Heiko Voigt <email@example.com> Signed-off-by: Shawn O. Pearce <firstname.lastname@example.org>