git-gui/gitk: Do not depend on Cygwin's "kill" command on Windows
Windows does not necessarily mean Cygwin, it could also be MSYS. The
latter ships with a version of "kill" that does not understand "-f". In
msysgit this was addressed shipping Cygwin's version of kill.
Properly fix this by using the stock Windows "taskkill" command instead,
which is available since Windows XP Professional.
Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>