context: raise ProgrammingError on repo['my-tag']

We had an internal extension that I had failed to migrate off of the
deprecated API and its "'my-tag' in repo" check just started returning
False. It took a while to figure out that that was what was
happening. This patch would have helped.

Differential Revision: https://phab.mercurial-scm.org/D3895

mercurial/context.py

@@ -432,6 +432,10 b' class changectx(basectx):'
                     raise
                 except (TypeError, LookupError):
                     pass
+            else:
+                raise error.ProgrammingError(
+                        "unsupported changeid '%s' of type %s" %
+                        (changeid, type(changeid)))
 
             # lookup failed
         except (error.FilteredIndexError, error.FilteredLookupError):