Add target with delegation

[jku]

Fixes #32

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Adding a target only modifies a single metadata so making it a "edit" sub command made sense... However, often user does not know which metadata should be modified: this is especially the case with succinct delegations.

Make add-target a top level action instead of a edit subaction:

• role can still be optionally specified but it is a starting point for the search
• delegation tree is followed from the starting role until the first leaf role (that does not delegate further) is found
• command now also outputs which role it inserted the target file into

This means add-target is now a simple repository wide command which seems quite logical: user does not need to know the details, they just want to add a target file to the repository.

There is a edit()-refactor here that makes the edit-contextmanager handle AbortEdit silently: the users of edit() can raise AbortEdit to signal they want to exit the contextmanager without saving the edited metadata. This is a bit undiscoverable and not immediately obvious but works quite nicely in both snapshot and add-target in my opinion.

[vmwclabot]

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[jku]

The obvious review comment here is that add-target and remove-target now look different... I have a commit for the latter as well, but would rather put that in another PR (it's not completely trivial as it makes sense to add a delegation search there as well).

[joshuagl]

@MVrachev care to take a look at this one?

[MVrachev]

Yes, I will. I will try to find time these days.

[vmwclabot]

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