[MERGE] Fix error handling in insert_data_stream (+ remove duplicate HeadsCache)

[Lukáš Lalinský]

Here is a patch for two problems in Repository.insert_data_stream:

   - It always fails on already present revisions for pack and remote
repositories, because they return parents as tuples and bdecode returns
lists.
   - It tries to use undefined RepositoryDataStreamError in case of
invalid stream item key.

(Btw, why is insert_data_stream in Repository? The implementation is
very knit-specific.)

There is one more revision in the bundle, but removing of the duplicate
HeadsCache is so trivial that I don't think it deserves a separate merge
request.

Lukas

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