merge nested functions which yield into single functions

Merge swh_hash_data_per_cvs_changeset() + process_cvs_changesets().
Merge swh_hash_data_per_cvs_rlog_changeset() + process_cvs_rlog_changesets().

Makes the code easier to follow and makes return values typed.

Also, cvs_changesets does not need to be a class member.
Make it a local variable instead. Pointed out by mypy.

Suggested by vlorentz.