statichttprepo: prevent loading dirstate over HTTP on node lookup (issue5717)

This seems a bit hacky, but works well. There should be no reason that
static-http repo had to load dirstate.

Initially I tried to proxy os.stat() call through vfs so that statichttpvfs
could hook it, but there wasn't a good error value which the statichttpvfs
could return to get around the util.filestat issue.

mercurial/context.py

@@ -471,7 +471,8 b' class changectx(basectx):'
                 self._node = repo.changelog.tip()
                 self._rev = repo.changelog.rev(self._node)
                 return
-            if changeid == '.' or changeid == repo.dirstate.p1():
+            if (changeid == '.'
+                or repo.local() and changeid == repo.dirstate.p1()):
                 # this is a hack to delay/avoid loading obsmarkers
                 # when we know that '.' won't be hidden
                 self._node = repo.dirstate.p1()
@@ -536,7 +537,8 b' class changectx(basectx):'
             #
             # XXX we could avoid the unfiltered if we had a recognizable
             # exception for filtered changeset access
-            if changeid in repo.unfiltered().dirstate.parents():
+            if (repo.local()
+                and changeid in repo.unfiltered().dirstate.parents()):
                 msg = _("working directory has unknown parent '%s'!")
                 raise error.Abort(msg % short(changeid))
             try:

tests/test-static-http.t

@@ -222,7 +222,6 b' List of files accessed over HTTP:'
   /.hg/bookmarks
   /.hg/bookmarks.current
   /.hg/cache/hgtagsfnodes1
-  /.hg/dirstate
   /.hg/requires
   /.hg/store/00changelog.i
   /.hg/store/00manifest.i
@@ -236,7 +235,6 b' List of files accessed over HTTP:'
   /remote-with-names/.hg/cache/branch2-served
   /remote-with-names/.hg/cache/hgtagsfnodes1
   /remote-with-names/.hg/cache/tags2-served
-  /remote-with-names/.hg/dirstate
   /remote-with-names/.hg/localtags
   /remote-with-names/.hg/requires
   /remote-with-names/.hg/store/00changelog.i
@@ -251,7 +249,6 b' List of files accessed over HTTP:'
   /remote/.hg/cache/hgtagsfnodes1
   /remote/.hg/cache/rbc-names-v1
   /remote/.hg/cache/tags2-served
-  /remote/.hg/dirstate
   /remote/.hg/localtags
   /remote/.hg/requires
   /remote/.hg/store/00changelog.i
@@ -268,7 +265,6 b' List of files accessed over HTTP:'
   /sub/.hg/bookmarks
   /sub/.hg/bookmarks.current
   /sub/.hg/cache/hgtagsfnodes1
-  /sub/.hg/dirstate
   /sub/.hg/requires
   /sub/.hg/store/00changelog.i
   /sub/.hg/store/00manifest.i