[MERGE] Improve the way we determine the signatures to return in item_keys_introduced_by

[Andrew Bennetts]

This basically just replaces a loop doing one-by-one get_signature_text
calls with a single get_parent_map call in item_keys_introduced_by.  John
and I also noticed that just hardcoding an empty result didn't break any
tests, so we changed the test suite to test that too.

Also, nothing at the moment actually uses this part of
item_keys_introduced_by's return value — instead fetch just asks for all
possible signatures for all the revisions it wants to fetch.  So
item_keys_introduced_by is doing work that is ignored.  We may want to
change item_keys_introduced_by to stop returning signatures, but this patch
at least keeps the current behaviour.  So at least we'll waste effort
efficiently :)

-Andrew.

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