context: set stack level for deprecation warning

This patch makes the deprecation warning print the caller of
repo.__getitem__. That's not quite correct, since there could also be
other callers of changectx.__init__, but it seems like the most
helpful stack level in practice.

Differential Revision: https://phab.mercurial-scm.org/D3307

mercurial/context.py

@@ -399,7 +399,7 b' def changectxdeprecwarn(repo):'
     #  * If "x" can be a mix of the above, you'll have to figure it out
     #    yourself
     repo.ui.deprecwarn("changectx.__init__ is getting more limited, see source "
-                       "for details", "4.6")
+                       "for details", "4.6", stacklevel=4)
 
 class changectx(basectx):
     """A changecontext object makes access to data related to a particular