arcconfig: stop amending commit message

[mchoo]

Summary

arcanist amends commit message by default when creating a revision, but
this is undesirable as FreeBSD has it's own commit message template.
Thus disable it for this repository through .arcconfig.

Test Plan

I verified that the commit message was not amended while creating this revision with arc diff --create.
I forgot to consider git-arc, so I'll hold this until I have time to check compatibility with git-arc.