git-gui: Don't nice git blame on MSYS as nice is not supported

git-gui: Don't nice git blame on MSYS as nice is not supported

Johannes Sixt reported that MinGW/MSYS does not have a nice.exe to
drop the priority of a child process when it gets spawned.  So we
have to avoid trying to start `git blame` through nice when we are
on Windows and do not have Cygwin available to us.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>