gitk: check main window visibility before waiting for it to show

If the main window is already visible when gitk waits for it to
become visible, gitk hangs forever.
This commit adds a check whether the window is already visible.
See https://wiki.tcl-lang.org/page/tkwait+visibility

Signed-off-by: Tobias Pietzsch <pietzsch@mycroft.speedport.ip>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>

Author: Tobias Pietzsch

gitk

@@ -12687,7 +12687,7 @@ catch {
     wm iconphoto . -default gitlogo gitlogo32
 }
 # wait for the window to become visible
-tkwait visibility .
+if {![winfo viewable .]} {tkwait visibility .}
 set_window_title
 update
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