git-gui--askyesno (mingw): use Git for Windows' icon, if available

This provides a unified look-and-feel in Git for Windows.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>

git-gui--askyesno

@@ -47,5 +47,17 @@ pack .b -side bottom -fill x -ipadx 20 -ipady 15
 bind . <Key-Return> {exit 0}
 bind . <Key-Escape> {exit 1}
 
+if {$::tcl_platform(platform) eq {windows}} {
+	set icopath [file dirname [file normalize $argv0]]
+	if {[file tail $icopath] eq {git-core}} {
+		set icopath [file dirname $icopath]
+	}
+	set icopath [file dirname $icopath]
+	set icopath [file join $icopath share git git-for-windows.ico]
+	if {[file exists $icopath]} {
+		wm iconbitmap . -default $icopath
+	}
+}
+
 wm title . $title
 tk::PlaceWindow .