exchange: switch to usual way of testing for bundle2-ness

We used safehasattr() in one place, but we use isinstance() for this
everywhere else, so switch to the latter.

mercurial/exchange.py

@@ -1731,7 +1731,7 b' def unbundle(repo, cg, heads, source, ur'
         # 'check_heads' call wil be a no-op
         check_heads(repo, heads, 'uploading changes')
         # push can proceed
-        if not util.safehasattr(cg, 'params'):
+        if not isinstance(cg, bundle2.unbundle20):
             # legacy case: bundle1 (changegroup 01)
             with repo.lock():
                 r = cg.apply(repo, source, url)