phases: write default phase data as soon as possible.

Marks phase data as dirty when computing default phase. Actual writing is done
when the lock is released. So, read only operation don't write phase data
because they don't lock the repo.

mercurial/phases.py

@@ -122,6 +122,7 b' def readroots(repo):'
             raise
         for f in repo._phasedefaults:
             roots = f(repo, roots)
+        repo._dirtyphases = True
     return roots
 
 def writeroots(repo):

tests/test-inherit-mode.t

@@ -121,6 +121,7 b' group can still write everything'
   00660 ../push/.hg/store/data/dir/bar.i
   00660 ../push/.hg/store/data/foo.i
   00660 ../push/.hg/store/fncache
+  00660 ../push/.hg/store/phaseroots
   00660 ../push/.hg/store/undo
   00660 ../push/.hg/store/undo.phaseroots
   00660 ../push/.hg/undo.bookmarks