metaedit: add support for folding commits while editing their metadata This also allows us to accept multiple commits without dealing with the thorny algorithmic and UI issues of editing multiple commits at once. Crucially, it is different from 'hg fold --exact' in that it also allows 'folding' a single commit and rewriting its metadata. This is really useful to have as a single logical operation, for example while preparing a series of multiple local changesets that will need to be pushed as a single changeset.

test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t