repoview: cache hidden changesets

Use the introduced caching infrastructure to cache hidden
changesets. We crosscheck if the content of the cache unless
experimental.verifyhiddencache is set to False. This will be removed
in the future. Without crosschecking the caches speed ups hg status and
other commands:

without caching:
$ time hg status
hg status  0.72s user 0.20s system 100% cpu 0.917 total

with caching
$ time hg status
hg status  0.49s user 0.15s system 100% cpu 0.645 total

mercurial/repoview.py

@@ -95,9 +95,9 b' def trywritehiddencache(repo, hideable, '
         fh.write(newhash)
         fh.write(data)
     except (IOError, OSError):
-        ui.debug('error writing hidden changesets cache')
+        repo.ui.debug('error writing hidden changesets cache')
     except error.LockHeld:
-        ui.debug('cannot obtain lock to write hidden changesets cache')
+        repo.ui.debug('cannot obtain lock to write hidden changesets cache')
     finally:
         if fh:
             fh.close()
@@ -128,12 +128,24 b' def computehidden(repo):'
 
     During most operation hidden should be filtered."""
     assert not repo.changelog.filteredrevs
+
     hidden = frozenset()
     hideable = hideablerevs(repo)
     if hideable:
         cl = repo.changelog
+        hidden = tryreadcache(repo, hideable)
+        if hidden is None:
         blocked = cl.ancestors(_getstaticblockers(repo), inclusive=True)
         hidden = frozenset(r for r in hideable if r not in blocked)
+            trywritehiddencache(repo, hideable, hidden)
+        elif repo.ui.configbool('experimental', 'verifyhiddencache', True):
+            blocked = cl.ancestors(_getstaticblockers(repo), inclusive=True)
+            computed = frozenset(r for r in hideable if r not in blocked)
+            if computed != hidden:
+                trywritehiddencache(repo, hideable, computed)
+                repo.ui.warn(_('Cache inconsistency detected. Please ' +
+                    'open an issue on http://bz.selenic.com.\n'))
+                hidden = computed
 
         # check if we have wd parents, bookmarks or tags pointing to hidden
         # changesets and remove those.