dpkg-source(1): add example of extend-diff-ignore's usage

Suggested-by: Christoph Berg <myon@debian.org>
Closes: #640198

debian/changelog

@@ -100,6 +100,8 @@ dpkg (1.16.1) UNRELEASED; urgency=low
     native formats in case of unexpected interruption. Closes: #631494
   * Fix simplification logic of union dependencies. Closes: #637564
   * Fix dpkg's handling of a hardlink pointing to a conffile. Closes: #638291
+  * Add example of extend-diff-ignore's usage in dpkg-source(1).
+    Closes: #640198
 
   [ Guillem Jover ]
   * Install deb-src-control(5) man pages in dpkg-dev. Closes: #620520

man/dpkg-source.1

@@ -695,6 +695,8 @@ Here's an example of such a file:
   compression\-level = 9
   # use debian/patches/debian\-changes as automatic patch
   single\-debian\-patch
+  # ignore changes on config.{sub,guess}
+  extend-diff-ignore = "(^|/)(config\.sub|config\.guess)$"
 .P
 Note: \fBformat\fR options are not accepted in this file, you should
 use \fBdebian/source/format\fR instead.