Improve "--hold", save/restore state, bookmark creation, and some warnings for "arc land" in Mercurial

Summary:
Ref T13546. Ref T9948.

• Make "--hold" show the same set of commands to manually push that the normal workflow would use.
• Make save/restore state work.
• Make bookmark creation prompt for confirmation.
• Improve / provide some additional warnings and help text.

Test Plan: Ran various increasingly complex "arc land" workflows, e.g. "arc land --hold --onto fauxmark1 --onto fauxmark2 --into default . --revision 118 --trace"

Maniphest Tasks: T13546, T9948

Differential Revision: https://secure.phabricator.com/D21351