gitk: Make .gitk a hidden file under windows

This sets the hidden attribute on the ~/.gitk file so it doesn't
appear in the windows user profile.
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -2519,6 +2519,9 @@ proc savestuff {w} {
     if {![winfo viewable .]} return
     catch {
 	set f [open "~/.gitk-new" w]
+	if {$::tcl_platform(platform) eq {windows}} {
+	    file attributes "~/.gitk-new" -hidden true
+	}
 	puts $f [list set mainfont $mainfont]
 	puts $f [list set textfont $textfont]
 	puts $f [list set uifont $uifont]