phase: extract the phaseroots serialization in a dedicated method

In most case, the file creation logic will be handled by the
transaction itself. The write method has to stay around for the case
where the repository is modified outside a transaction (strip).

mercurial/phases.py

@@ -196,11 +196,14 b' class phasecache(object):'
             return
         f = self.opener('phaseroots', 'w', atomictemp=True)
         try:
+            self._write(f)
+        finally:
+            f.close()
+
+    def _write(self, fp):
             for phase, roots in enumerate(self.phaseroots):
                 for h in roots:
-                    f.write('%i %s\n' % (phase, hex(h)))
-        finally:
-            f.close()
+                fp.write('%i %s\n' % (phase, hex(h)))
         self.dirty = False
 
     def _updateroots(self, phase, newroots):