gc: Support multiple --rev.

[daavoo]

What if I wanted to do something like dvc gc --rev HEAD --rev baseline --rev some_other_tag? That would depart from how --rev is used in exp commands and is a separate request, but I'm wondering if we are making that future possibility harder by introducing a different --rev behavior (related to #5066, which asks for skipping specific revs). Not a blocker, just a random thought.

Originally posted by @dberenbaum in #7971 (comment)