git-gui: ensure correct application termination in git-gui--askpass

With Tk 8.5 the askpass utility can hang waiting for the wish shell
implicit event loop to exit. This patch uses an explicit event loop
to ensure correct application termination.

Reported-by: Anders Kaseorg <andersk@mit.edu>
Tested-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

git-gui--askpass

@@ -5,6 +5,8 @@ exec wish "$0" -- "$@"
 # This is a trivial implementation of an SSH_ASKPASS handler.
 # Git-gui uses this script if none are already configured.
 
+package require Tk
+
 set answer {}
 set yesno  0
 set rc     255
@@ -30,16 +32,20 @@ if {!$yesno} {
 
 frame .b
 button .b.ok     -text OK     -command finish
-button .b.cancel -text Cancel -command {destroy .}
+button .b.cancel -text Cancel -command cancel
 
 pack .b.ok -side left -expand 1
 pack .b.cancel -side right -expand 1
 pack .b -side bottom -fill x -padx 10 -pady 10
 
 bind . <Visibility> {focus -force .e}
-bind . <Key-Return> finish
-bind . <Key-Escape> {destroy .}
-bind . <Destroy>    {exit $rc}
+bind . <Key-Return> [list .b.ok invoke]
+bind . <Key-Escape> [list .b.cancel invoke]
+bind . <Destroy>    {set rc $rc}
+
+proc cancel {} {
+	set ::rc 255
+}
 
 proc finish {} {
 	if {$::yesno} {
@@ -50,10 +56,11 @@ proc finish {} {
 		}
 	}
 
-	set ::rc 0
 	puts $::answer
-	destroy .
+	set ::rc 0
 }
 
 wm title . "OpenSSH"
 tk::PlaceWindow .
+vwait rc
+exit $rc