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
#require docutils gettext
$ $TESTDIR/check-gendoc de
checking for parse errors
Die Dateien werden dem Projektarchiv beim n\xc3\xa4chsten \xc3\x9cbernehmen (commit) hinzugef\xc3\xbcgt. Um dies vorher r\xc3\xbcckg\xc3\xa4ngig zu machen, siehe:hg:`forget`. (esc)
warning: please have a space before :hg: