detect whether commit A depends on commit B

It would be useful to be able to feed `git-deps` two commit-ishes and have it exit 0 if and only if the first depends on the second.

Start by tracing how git-deps accepts commit-ish arguments and computes dependency relationships. Completion means accepting two commits and exiting 0 exactly when the first depends on the second, with the opposite status otherwise; the issue identifies no file, entry point, or test path.

Written by the indexing model from the issue text.