fix: Public reconciliation of a concurrent write and remove (of independent paths)

[matheus23]

This is an edge case I came across while implementing private conflict reconciliation.

Essentially we have this setup:

• Both replicas share the same state of a single file, e.g. at "public/a/b.txt".
• Then one replica deletes that file
• The other replica does an unrelated write, e.g. it adds "public/file.txt"
• Both replicas merge again

What happened previously was:

• We detected that two independent writes were done on the root directory
• We run a deep merge on both directories. The b.txt file gets re-introduced (the remove gets "overwritten"), because we don't re-check the causal relationship of the "a" directories.

We now check the causal relationship between directories on every level (recursively).

I also removed the merge function, as otherwise it'd be hard to disentangle the recursion, but that'd be needed to not duplicate the whole reconcile_helper and merge_helper functions.