Add support for (globally) to cross-edit Commits

[osnr]

If program virtual-programs/a.folk does

Commit hello { Claim hello }

then another program can do

Commit (globally) virtual-programs/a.folk hello { Claim goodbye }

to mutate that Commit.

Useful for hooking into the editor.

[nmichaud]

Was just going to comment, how about this phrasing:

Commit on virtual-programs/a.folk, hello {Claim goodbye}?