Ensure that binNMUs from superseded sources are not considered

Given a source which provides two packages and has different versions
in testing and unstable, binNMUs in unstable corresponding to the
older source version should not be considered as migration candidates.

For example:

testing
-------

source 1
bin 1 arch1
bin 1 arch2

unstable
--------

source 2
bin 2 arch1
bin 1+b1 arch2

The binary migration on arch2 should not be considered a candidate.

Signed-off-by: Adam D. Barratt <adam@adam-barratt.org.uk>

britney.py

@@ -1187,6 +1187,12 @@ class Britney(object):
                 excuse.addhtml("From wrong source: %s %s (%s not %s)" % (pkg_name, binary_u[VERSION], pkgsv, source_t[VERSION]))
                 break
 
+            # if the source package has been updated in unstable and this is a binary migration, skip it
+            if self.same_source(source_t[VERSION], pkgsv) and source_t[VERSION] != source_u[VERSION]:
+                anywrongver = True
+                excuse.addhtml("From wrong source: %s %s (%s not %s)" % (pkg_name, binary_u[VERSION], pkgsv, source_u[VERSION]))
+                break
+
             # find unsatisfied dependencies for the new binary package
             self.excuse_unsat_deps(pkg_name, src, arch, suite, excuse)