git-gui: provide question helper for retry fallback on Windows

Make use of the new environment variable GIT_ASK_YESNO to support the
recently implemented fallback in case unlink, rename or rmdir fail for
files in use on Windows. The added dialog will present a yes/no question
to the the user which will currently be used by the windows compat layer
to let the user retry a failed file operation.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>

git-gui/git-gui--askyesno

@@ -0,0 +1,51 @@
+#!/bin/sh
+# Tcl ignores the next line -*- tcl -*- \
+exec wish "$0" -- "$@"
+
+# This is an implementation of a simple yes no dialog
+# which is injected into the git commandline by git gui
+# in case a yesno question needs to be answered.
+
+set NS {}
+set use_ttk [package vsatisfies [package provide Tk] 8.5]
+if {$use_ttk} {
+	set NS ttk
+}
+
+if {$argc < 1} {
+	puts stderr "Usage: $argv0 <question>"
+	exit 1
+} else {
+	set prompt [join $argv " "]
+}
+
+${NS}::frame .t
+${NS}::label .t.m -text $prompt -justify center -width 40
+.t.m configure -wraplength 400
+pack .t.m -side top -fill x -padx 20 -pady 20 -expand 1
+pack .t -side top -fill x -ipadx 20 -ipady 20 -expand 1
+
+${NS}::frame .b
+${NS}::frame .b.left -width 200
+${NS}::button .b.yes -text Yes -command yes
+${NS}::button .b.no  -text No  -command no
+
+
+pack .b.left -side left -expand 1 -fill x
+pack .b.yes -side left -expand 1
+pack .b.no -side right -expand 1 -ipadx 5
+pack .b -side bottom -fill x -ipadx 20 -ipady 15
+
+bind . <Key-Return> {exit 0}
+bind . <Key-Escape> {exit 1}
+
+proc no {} {
+	exit 1
+}
+
+proc yes {} {
+	exit 0
+}
+
+wm title . "Question?"
+tk::PlaceWindow .