git-gui/gitk: Do not use a Cygwin-specific kill flag 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>
Merge pull request #239 from sschuberth/taskkill

git-gui/gitk: Do not use a Cygwin-specific kill flag on Windows