make-release: Fix --no-previous

It didn't actually work since nothing was checking it.

Change-Id: I3be73ec6f68b96ff8b3d40316c3b779e203792f3
make-release: Rename and document some functions and add a test

Use lowercase and underscores in the function names per python
conventions, and do the same for variables.

Write a test too!

Change-Id: Id0e89ef6a2e152b451c1be947fc3dba1f76d33be