Don't flag unbuilt binary *pu binNMUs for removal in excuses

Originally when binNMUs for packages in testing were scheduled, the
binaries would be installed into tpu with no accompanying source. This
allowed the "removed binary" portions of should_upgrade_srcarch() to be
skipped (as britney had generated a faux source record).

dak now adds the source package to tpu in such cases which lead to the
"removed binary" checks being applied to binNMUs in tpu with potentially
destructive consequences. For example, if a package with amd64 and i386
binaries in testing were binNMUed on just amd64, britney would notice
that there were no i386 binaries in tpu and subsequently remove the i386
binaries from testing as well.

In order to resolve this, we skip the check for removed binaries when
building excuses for a binary-only migration via *pu.

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

britney.py

@@ -1213,6 +1213,9 @@ class Britney(object):
         if not anywrongver and (anyworthdoing or not self.sources[suite][src][FAKESRC]):
             srcv = self.sources[suite][src][VERSION]
             ssrc = self.same_source(source_t[VERSION], srcv)
+            # if this is a binary-only migration via *pu, we never want to try
+            # removing binary packages
+            if not (ssrc and suite != 'unstable'):
                 # for every binary package produced by this source in testing for this architecture
                 for pkg in sorted([x.split("/")[0] for x in self.sources['testing'][src][BINARIES] if x.endswith("/"+arch)]):
                     # if the package is architecture-independent, then ignore it