strip: access bookmark before getting a reference to changelog

Bookmark access might invalidate the current changelog (to make sure both are in
a reasonable synchronisation state). So we should grab the reference to
changelog after we access bookmark. Otherwise we risk using a dead object for
the whole strip process.

(note: this dead object business probably requires a new layers of checking)

mercurial/repair.py

@@ -109,6 +109,9 b' def strip(ui, repo, nodelist, backup=Tru'
     repo = repo.unfiltered()
     repo.destroying()
     vfs = repo.vfs
+    # load bookmark before changelog to avoid side effect from outdated
+    # changelog (see repo._refreshchangelog)
+    repo._bookmarks
     cl = repo.changelog
 
     # TODO handle undo of merge sets