gitk: Force the focus to the main window on Windows

On msysGit, the focus is first on the (Tk) console.  This console is then
hidden, but keeps the focus.  Work around that by forcing the focus onto
the gitk window.

This fixes msysGit issue 14.  Diagnosed and originally fixed by
Johannes Schindelin.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -10914,4 +10914,9 @@ if {[info exists permviews]} {
 	addviewmenu $n
     }
 }
+
+if {[tk windowingsystem] eq "win32"} {
+    focus -force .
+}
+
 getcommits {}