add single-debian-patch to debian/source/options

Debian/squeeze's dpkg-source automatically commits changes:

| dpkg-source: info: local changes stored in source/debian/patches/debian-changes-2.1.3-7~sipwise3+0~20130624111524.76+wheezy~1.gbp21dd0b, the modified files are:

whereas that's no longer true for Debian/wheezy:

| dpkg-source: error: aborting due to unexpected upstream changes, see /tmp/heartbeat-2_2.1.3-7~sipwise3+0~20130708093100.78+wheezy~1.gbp21dd0b.diff.n4aEQk
| dpkg-source: info: you can integrate the local changes with dpkg-source --commit

In heartbeat_2.1.3.orig.tar.gz (md5: bca53530a3802f7677772323047405cd)
the debian/ directory was present already and some *changes* to
it as well as changes to upstream code (sic!) have been part of
heartbeat_2.1.3-8.diff.gz then.

Let's automatically commit those changes through dpkg-source instead
of manually adding them to a debian/patches/ patch, at least
until it can be easily resolved through a new upstream version
instead of rewriting upstream branch/tag history.

Addressing build error https://jenkins.mgm.sipwise.com/job/heartbeat-source/78/console

debian/source/options

@@ -0,0 +1 @@
+single-debian-patch