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
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.