Allow the user to select exactly which references to include in the analysis

[mhagger]

It would often be convenient to select exactly which references should be used as starting points when enumerating objects for analysis. We already have --branches, --tags, and --remotes to cause those types of references to be included. But what about

• Include all references except for those under refs/notes (see issue #1).
• Include all references except for the special refs/stash reference used for managing stashes.
• Include all references except for remote references and GitHub's special refs/pulls references.
• Include all branches and tags but not CI build tags that match refs/tags/build-*.

This PR adds four new options that make it possible to select arbitrary subsets of references:

• --include PREFIX and --include-regexp PATTERN: specify references that should be included in the analysis.
• --exclude PREFIX and --exclude-regexp PATTERN: specify references that should not be included in the analysis.

I still need to add tests.

[mhagger]

This could use more integration testing, but it does have unit tests and I've tested it manually, so I think it's GTG.

[mhagger]

I added an integration test after all. Wenn schon, denn schon.