This is a thin wrapper around `arc patch` that attempts to produce
a commit message that more closely resembles the commit template.
This can also be done by patching the phabricator installation (this
has been done e.g. by LLVM), but a separate script seems like the
simpler solution.
Updating the commit message is done by a separate script, arcfilter.sh
which could also be used by git-arc. That script is a copy of
llvm/utils/git/arcfilter.sh in the LLVM repository with a few minor changes.