change find_ids_across_trees to return None when asked to find ids for ['']

[Robert Collins]

find_ids_across_trees currently returns all the ids in all trees when
asked to find ids for the path ['']. This results in double handling of
id data when we dont need to.

find_ids_across_trees should be changed to return None when filenames is
['']: but this will require changing code that currently does not handle
None in this case.

-Rob
 affects /products/bzr
 status confirmed
 importance medium
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[Aaron Bentley (abentley)]

I think this is totally bogus. find_ids_across_trees should only return None if its input is None.

Functions and methods that get an empty list should treat it as an empty list. I think you want to treat user input of the empty list as meaning wildcard, but that should be done as part of user input parsing, and perhaps not in all cases.

Treating [] differently from all other lists is a bogosity that does not belong in the Bazaar API, e.g. WT.revert