Summary:
Currently, we throw a fairly perplexing error when there are multiple valid
commit messages. Installs can also remove the "test plan" field entirely, which
is the only really strong discriminator here.
When the message to use is ambiguous, show the user all the valid messages and
prompt them to choose one.
Also add a -C flag like "git commit -C", so they can choose a message
explicitly.
Test Plan: Ran "arc diff HEAD^^^^^", "arc diff -C <rev>".
Reviewers: cpiro, btrahan, jungejason
Reviewed By: cpiro
CC: aran, cpiro
Differential Revision: https://secure.phabricator.com/D1385
f271e1d8e8