core supports arbitrary revision names, but no user-facing API

[jwise]

The core supports arbitrary revision names in core.v1.entities.cut_revision, but the only callsite for cut_revision is in bump_revision, which always takes its revision label from _compute_next_label_from_fs. This doesn't work so well for my use case, where my internal revision numbering is [letter] [number*2] (i.e., 'A01', 'B01', 'B02', ...).

It would be good for the CLI and web UI to be able to specify other revision names.