devel-warn: issue a warning when writing bookmarks without holding the wlock

I saw an issue in an extension that we develop where we were writing bookmarks
without holding the wlock. Another extension was taking a lock at the same time
and wiped out the bookmarks we were about to write. This patch adds a
devel-warning to urge people to fix their invalid code.

mercurial/bookmarks.py

@@ -90,6 +90,11 b' class bmstore(dict):'
         can be copied back on rollback.
         '''
         repo = self._repo
+        if (repo.ui.configbool('devel', 'all-warnings')
+                or repo.ui.configbool('devel', 'check-locks')):
+            l = repo._wlockref and repo._wlockref()
+            if l is None or not l.held:
+                repo.ui.develwarn('bookmarks write with no wlock')
         self._writerepo(repo)
         repo.invalidatevolatilesets()