Remove history.immutable from .arcconfig

The history.immutable setting prevents arcanist from updating
the commit messages with the Differential URL and therefore
makes updating patches awkward with a rebase workflow.

In case this new behaviour is not wanted the old one can be restored
by running arc set-config --local history.immutable true.

Test Plan: arc diff --create HEAD^ adds the metadata now.

Reviewed By: phabric-admin, imp, lwhsu
Differential Revision: https://reviews.freebsd.org/D27971