Pass conduit credentials down to children workflow

Summary:
Calling `arc patch` on a diff that's dependent on a different diff tries to patch the parent first.
To patch the parent a child workflow is created, and a conduit is passed down, but the credentials are not and it is not marked as authenticated.

Then when the child tries to get the commit message for the dependency, it checks isConduitAuthenticated() https://secure.phabricator.com/diffusion/ARC/browse/master/src/workflow/ArcanistPatchWorkflow.php;2c3268f03ed70d3221eb1642bIc99ebb39b12902e$800 and on failure pops up an interactive editor for the commit message.

Instead we just pass down the credentials to the childred and mark them as authenticated, so this is not a problem.

Test Plan: With two diffs where DA2 depends on DA1, run `arc patch --force --nobranch DA2` ... this no longer pops an interactive editor for the commit message for the dependency.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: hach-que, seshness, epriestley, Korvin

Maniphest Tasks: T5986

Differential Revision: https://secure.phabricator.com/D10381
restored --head support

Summary: the call to setHeadCommit() was accidentally placed in a `if ($background) {}` block, which was removed in 54bea94. This adds the removed call.

Test Plan: used --head, worked as expected

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10447
Patch: don't complain about reviewers not being specified.
This will break in the future when upstream arc incorporates T4631. Hopefully there will be a better solution by then
if you didn't specify a reviewer, suggest projects