convert: added bookmarks support in filemap

When using the convert extension from a Mercurial rep. to subset it with
filemap, the bookmarks are not copied. I fixed this by calling the
base.get_bookmarks() from the filemap getbookmarks() instead of returning an
empty dictionary. It should work also for other converters that implement
getbookmarks() (like git). I don't see any drawbacks except that the bookmarks
are always copied (not necessarily wanted all the times).

hgext/convert/filemap.py

@@ -375,3 +375,6 b' class filemap_source(converter_source):'
 
     def lookuprev(self, rev):
         return self.base.lookuprev(rev)
+
+    def getbookmarks(self):
+        return self.base.getbookmarks()