limit delete.namespace output

[ceedubs]

Per Slack discussion.

Say that I do something like fork .base .base2 and then delete.namespace .base2. Currently the delete.namespace takes a while to run (~minute). Part of this time (though we haven't measured how much) is spent rendering a pretty-printed description of what has changed. This output is verbose and not that useful (and possibly taking a long time to render).

@pchiusano's proposal:

Print out the names (no type signatures needed) that were removed, up to 20 or whatever then add some ellipses if there were more.