add: hash-only: set the prefix for MFS root

[schomatis]

In the case that the only-hash option (-n) is set for ipfs add, use the same prefix from the fileAdder for the MFS root so it is processed using the same CID version and hash function.

Fixes #4652.

This is a work in progress, some tests should be added. Also, the comment is not very illustrative, it should clarify why is it that the prefix needs two be specified twice (for the file adder and the MFS root).

[djdv]

This works on my node. 👍

A simple test case you can add for this one, is to capture the hashes generated by any add parameter set and compare them with the same set + -n, if they do not match the test should fail, otherwise it should pass.

e.g.
The hashes produced by add -w --cid-version=1 "testdata" must equal the output hashes produced by add -n -w --cid-version=1 "testdata".
The hashes produced by add -w --hash=Blake2b-256 "testdata" must equal the output hashes produced by add -n -w --hash=Blake2b-256 "testdata".

[schomatis]

@kevina I based this test on your ipfs add -w -r (dir) (adding the -n option), as the issue was occurring for CIDv1, WDYT?

[schomatis]

@Stebalien In this PR the prefix used for the fileAdder is also applied to the MFS root, to ensure consistency across the file hashes. Is this correct?

The MFS root is the only node that isn't added by the fileAdder (because it's already there before the addition of the user files), does that make sense? I want to add an explanation (in the form of a comment) as to why the prefix has to be referenced twice.

[Kubuxu]

Tanks for including tests.

[whyrusleeping]

Another one, thanks @schomatis!