Add a difftool that ignores frequently changing lines

Usage:

    git difftool --dir-diff --extcmd=tools/diff-interesting.sh

tools/diff-interesting.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+# Usage:
+#
+#     git difftool --dir-diff --extcmd=tools/diff-interesting.sh
+#
+diff --recursive --unified=1 --color \
+     --ignore-matching-lines=serverAddress \
+     --ignore-matching-lines='^\*  subject:' \
+     --ignore-matching-lines='^\*  start date:' \
+     --ignore-matching-lines='^\*  expire date:' \
+     --ignore-matching-lines='^\*  issuer:' \
+     --ignore-matching-lines='^< Date:' \
+     --ignore-matching-lines='^< Content-Length:' \
+     --ignore-matching-lines='--:--:--' \
+     --ignore-matching-lines='{ \[[0-9]* bytes data\]' \
+     "$@"