phase: isolate logic to update remote phrase through bundle2 pushkey

Move the logic to build bundle2 pushkey part into its dedicated function. It
will help to keep the logic clear when adding support for sending phases change
using 'phase-heads' part.

mercurial/exchange.py

@@ -806,8 +806,11 def _pushb2phases(pushop, bundler):
     if 'phases' in pushop.stepsdone:
         return
     b2caps = bundle2.bundle2caps(pushop.remote)
-    if not 'pushkey' in b2caps:
-        return
+    if 'pushkey' in b2caps:
+        _pushb2phasespushkey(pushop, bundler)
+
+def _pushb2phasespushkey(pushop, bundler):
+    """push phase information through a bundle2 - pushkey part"""
     pushop.stepsdone.add('phases')
     part2node = []