add `resolve` command

[Rudxain]

It works like this:

git diff --name-only --diff-filter=U --relative -z |
    xargs -0r $(git var GIT_EDITOR) && \
    git add --ignore-errors . && \
    git continue

I have an approximation of it as an alias

[spacewander]

@hyperupcall
What do you think?

[hyperupcall]

If I'm understanding this correctly, this handles all merge conflicts by launching each file in an editor, where the user can resolve the conflict, right? It might seem useful. Any reason why the --ignore-errors is needed?