pull: pre-filter remote phases before moving local ones

We were relying on the phase internals to filter out redundant phase
information from remove. However as we plan to integrate phase
movement inside the transaction, we want to avoid useless transaction
creation on no-op pulls.

Therefore we filter out all the information that already matches the current
repository state. This will let us create a transaction only when there is
actual phase movement needed.

mercurial/exchange.py

@@ -825,14 +825,27 b' def _pullapplyphases(pullop, remotephase'
         pheads, _dr = phases.analyzeremotephases(pullop.repo,
                                                  pullop.pulledsubset,
                                                  remotephases)
-        phases.advanceboundary(pullop.repo, phases.public, pheads)
-        phases.advanceboundary(pullop.repo, phases.draft,
-                               pullop.pulledsubset)
+        dheads = pullop.pulledsubset
     else:
         # Remote is old or publishing all common changesets
         # should be seen as public
-        phases.advanceboundary(pullop.repo, phases.public,
-                               pullop.pulledsubset)
+        pheads = pullop.pulledsubset
+        dheads = []
+    unfi = pullop.repo.unfiltered()
+    phase = unfi._phasecache.phase
+    rev = unfi.changelog.nodemap.get
+    public = phases.public
+    draft = phases.draft
+
+    # exclude changesets already public locally and update the others
+    pheads = [pn for pn in pheads if phase(unfi, rev(pn)) > public]
+    if pheads:
+        phases.advanceboundary(pullop.repo, public, pheads)
+
+    # exclude changesets already draft locally and update the others
+    dheads = [pn for pn in dheads if phase(unfi, rev(pn)) > draft]
+    if dheads:
+        phases.advanceboundary(pullop.repo, draft, dheads)
 
 def _pullobsolete(pullop):
     """utility function to pull obsolete markers from a remote