rev-list: teach --no-object-names to enable piping
Allow easier parsing by cat-file by giving rev-list an option to print
only the OID of a non-commit object without any additional information.
This is a short-term shim; later on, rev-list should be taught how to
print the types of objects it finds in a format similar to cat-file's.
Before this commit, the output from rev-list needed to be massaged
before being piped to cat-file, like so: