unbundle: swap conditional branches for clarity

This is a small style update for clarity. The previous situation was:

  if foo:
    50 lines
  else:
    2 lines

In such case I tend to invert these to get the simpler branch out of the way
earlier:

  if not foo:
    2 lines
  else:
    50 lines

This makes the conditional and various alternatives fit on the same screen,
simpler to read overall.

mercurial/exchange.py

@@ -1723,7 +1723,11 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 util.safehasattr(cg, 'params'):
+        if not util.safehasattr(cg, 'params'):
+            # legacy case: bundle1 (changegroup 01)
+            lockandtr[1] = repo.lock()
+            r = cg.apply(repo, source, url)
+        else:
             r = None
             try:
                 def gettransaction():
@@ -1762,10 +1766,6 b' def unbundle(repo, cg, heads, source, ur'
                                                   mandatory=False)
                         parts.append(part)
                 raise
-        else:
-            # legacy case: bundle1 (changegroup 01)
-            lockandtr[1] = repo.lock()
-            r = cg.apply(repo, source, url)
     finally:
         lockmod.release(lockandtr[2], lockandtr[1], lockandtr[0])
         if recordout is not None: