gitk: On Windows, use a Cygwin-specific flag for kill

MSysGit compiles git binaries as native Windows executables,
so they cannot be killed unless a special flag is specified.

This flag is implemented by the Cygwin version of kill,
which is also included in MSysGit.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -388,8 +388,13 @@ proc stop_instance {inst} {
     set fd $commfd($inst)
     catch {
 	set pid [pid $fd]
+
+	if {$::tcl_platform(platform) eq {windows}} {
+	    exec kill -f $pid
+	} else {
 	    exec kill $pid
 	}
+    }
     catch {close $fd}
     nukefile $fd
     unset commfd($inst)