`delegate_hashed_bins` must hash a target path as it appears in metadata

Description of issue or feature request: Fix in accordance with https://github.com/theupdateframework/tuf/issues/957 (and #963)!!

The repository tool’s delegate_hashed_bins creates hash bins (delegated targets roles) and assigns passed target file paths based on their hash. Later it calls delegate for each bin passing the relevant paths. delegate then might left-strip the targets directory path from each targets path, changing the targets path in a way that it no longer is in the right bin.

Current behavior: delegate_hashed_bins might hash target path based on a different path than the one that will appear in the (delegated) targets metadata.

Expected behavior: delegate_hashed_bins must hash a target path as it appears in metadata.

[lukpueh]

@trishankatdatadog, you were right. The variables in the division were switched plus two other minor issues (see edit history of comment). For my defense I only quickly wrote this up today from some pen and paper notes I took last week. Apologies. 😃 You should be able to copy-paste the updated snippet into your interpreter now and it should show that it works.

[lukpueh]

Also, as hinted above, this snippet should not be copy-pasted as is. It will need some fleshing out to replace the existing functionality in repository_tool. I just wanted to outline the algorithm I had in mind, for distributing targets in an ordered list of bins, where the index equals the bin size divided by the lower bound of the hash prefix range served by the bin.