[PATCH 0 of 3 hg-bfiles] Refactoring of stores' get (take 2)

[david.douard at logilab.fr]

Hi Greg and others,

here a reworked copy of the refactoring (plus the hash verification) of the store's get method to have more factorized code. 

I tried to apply your advices, however, there is still the "Blecch" about a function closing a file it did not opened. 

I modified a bit the _copy_and_hash API to make it easily usable with the sshstore. 

I also made the hash check verification of the get method a new changeset to let the refactoring changeset a pure refactoring one. I added a test in test-verify. Not sure it's the best place for it, but it's the place it was easier to add this test.

Hope it'll be OK.

David