arcconfig: turn on mutable history
arcconfig: turn on mutable history

Summary:
By turning on mutable history, `arc land` will squash a branch's history
to a single commit.  This works better with commit audits since
Phabricator checks each individual commit against a revision's last diff
to determine whether to create an audit.  With immutable history, this
means that any review changes added as a separate commit would trigger
an audit.

More information about History Mutablitity can be found here:
https://secure.phabricator.com/book/phabricator/article/arcanist_new_project/#history-mutability

Reviewers: fromberger, zarko, jdennett

Reviewed By: jdennett

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D123

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