Improve handling of commit message on --auto/--update/--create workflows

Summary:
Our failure behavior currently sucks. In particular:

• If lint or unit fail (expectedly or unexpectedly), we throw away your

message. omglol sux 2 b u

• If there's a parsing error, we dump the message to 'arc-commit-message' in

CWD and tell you to go deal with it.

This is awful. Improve the behavior:

• Once we read a message, save it in .arc/commit-message so we always have it

if anything goes wrong.

• If that file exists, prompt to read it without any "-F" nonsense.
• If there's a parsing error, tell the user and prompt them to just edit the

file again, showing what they need to fix.

Test Plan: Ran "arc diff --auto" and made a bunch of intentional errors; arc
never screwed me over and was generally fairly pleasant.

Reviewers: btrahan, Makinde

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T614, T895

Differential Revision: https://secure.phabricator.com/D1656