Allow combining multiple items with but rub

[rohitpaulk]

Often find myself trying to merge two changed files into an existing commit, and I have to do the following:

• but status to view IDs
• but rub <file1> <commit1> to add the first file to the commit
• but status again, since IDs change every time
• but rub <file2> <commit2> to add the second file to the commit

Wonder if but rub could just take multiple sources? but rub <file1> <file2> ... <commit1>.

I don't think there's a use-case for multiple targets, so this could be done without flags / complicating the interface.

[Byron]

That's a great idea, thanks a lot for sharing!

And I agree, despite multiple items, as long as the last item is the target, it should still be pretty unambiguous to what should be done.
Another thought here is if hashes can be more stable, so probably be based more on the content. This might make them longer, but the UI could still highlight their unique prefixes to trim them to one or two characters in the common cases.

CC @krlvi in case we are missing something.