pull: move phases synchronisation in its own function

Now that every necessary information is held in the `pulloperation` object, we
can finally extract the phase synchronisation phase to it's own function.

This changeset is pure code movement only.

mercurial/exchange.py

@@ -473,6 +473,16 b' def pull(repo, remote, heads=None, force'
             subset = pullop.heads
         pullop.pulledsubset = subset
 
+        _pullphase(pullop)
+        _pullobsolete(pullop)
+        pullop.closetransaction()
+    finally:
+        pullop.releasetransaction()
+        lock.release()
+
+    return result
+
+def _pullphase(pullop):
         # Get remote phases data from remote
         remotephases = pullop.remote.listkeys('phases')
         publishing = bool(remotephases.get('publishing', False))
@@ -490,14 +500,6 b' def pull(repo, remote, heads=None, force'
             phases.advanceboundary(pullop.repo, phases.public,
                                    pullop.pulledsubset)
 
-        _pullobsolete(pullop)
-        pullop.closetransaction()
-    finally:
-        pullop.releasetransaction()
-        lock.release()
-
-    return result
-
 def _pullobsolete(pullop):
     """utility function to pull obsolete markers from a remote