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>

git-gui/git-gui.sh

@@ -666,9 +666,7 @@ proc kill_file_process {fd} {
 
 	catch {
 		if {[is_Windows]} {
-			# Use a Cygwin-specific flag to allow killing
-			# native Windows processes
-			exec kill -f $process
+			exec taskkill /pid $process
 		} else {
 			exec kill $process
 		}