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 by 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: NSebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -445,7 +445,7 @@ proc stop_instance {inst} {
 	set pid [pid $fd]
 
 	if {$::tcl_platform(platform) eq {windows}} {
-	    exec kill -f $pid
+	    exec taskkill /pid $pid
 	} else {
 	    exec kill $pid
 	}