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>
git-gui: fix verbose loading when git path contains spaces.

After setting GITGUI_VERBOSE to enable load tracing the source command is
redefined but fails if the git installation path has spaces. Fixed quoting
by using lists to handle the arguments.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>