tests/test-debugindexdot.out

rebase: improve help text

- add a paragraph about default dest/source changesets
- option help: say "changeset" not "revision"
- option help: explain -b/--base better
- clarify that -a/--abort and -c/--continue are different from the
  other options

created new head
digraph G {
	-1 -> 0
	0 -> 1
	0 -> 2
	2 -> 3
	1 -> 3
}