gitk: check main window visibility before waiting for it to show (#705)

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

This ports git#944 to `microsoft/git`. for
Git.

Fixed issue #704.

Author: dscho

gitk-git/gitk

@@ -12832,7 +12832,7 @@ if {$::tcl_platform(platform) eq {windows} && [file exists $gitk_prefix/etc/git.
     }
 }
 # wait for the window to become visible
-tkwait visibility .
+if {![winfo viewable .]} {tkwait visibility .}
 set_window_title
 update
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