contrib: add a script to generate contrib patches

Usage in the specific contrib folder you're building:
../diffpatch.sh <folder_of_package> <file_extension_between_old_new_versions>

eg
../diffpatch.sh flac .mingw-fix

contrib/diffpatch.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+# create a patch file based on file extension
+# $1 folder to run the find in
+# $2 extension to use for the diff
+find $1 -name "*$2" -exec bash -c 'filepath="$1" ; dir=$(dirname "$filepath") ; base=$(basename -s $2 "$filepath") ; diff -pur $filepath $dir/$base' -- {} $2 \;