Add --commit to automated-commits to make local testing easier

Summary:
Local testing of automated-commits can be made difficult because it currently
assumes building on top of origin/master. The best example of this is working on a stack
of changes locally, where the top of the stack is a change to automated-commits itself
that depends on commits not yet landed on master.

This patch fixes that by enabling the selection of a commit to build on top of other than
latest master.

Test Plan:

arc patch D5270

git checkout master
git checkout -b testbranch
git cherry-pick arcpatch-D5270  # only take the changes from the D5270 patch, not its parent (this patch)
COMMIT_TYPE=update-chainparams ./automated-commits.sh  # fails

git checkout arcpatch-D5270
./automated-commits.sh -h
COMMIT_TYPE=update-chainparams ./automated-commits.sh --parent HEAD  # succeeds
DRY_RUN=no COMMIT_TYPE=update-chainparams ./automated-commits.sh --parent HEAD  # fails

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5269