convert: compare tags from all heads instead of just one

Previously, the hg sink for puttags would just use one head for getting the old
tags which would sometimes lead to tags disappearing. Now, we iterate over all
heads and merge the results.

hgext/convert/hg.py

@@ -201,10 +201,13 b' class mercurial_sink(converter_sink):'
             parentctx = None
             tagparent = nullid
 
-        try:
-            oldlines = sorted(parentctx['.hgtags'].data().splitlines(True))
-        except Exception:
-            oldlines = []
+        oldlines = set()
+        for branch, heads in self.repo.branchmap().iteritems():
+            for h in heads:
+                if '.hgtags' in self.repo[h]:
+                    oldlines.update(
+                        set(self.repo[h]['.hgtags'].data().splitlines(True)))
+        oldlines = sorted(list(oldlines))
 
         newlines = sorted([("%s %s\n" % (tags[tag], tag)) for tag in tags])
         if newlines == oldlines: