Support multiple stable release branches / consider all branches when looking for previous release shas

[trevorwhitney]

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Fixes #2202 🦕

We are currently using this fork for our release process and it appears to be working as expected, but I'm sure there are edge cases that aren't being addressed yet.

This PR add a flag --consider-all-branches that, when specified, will use GitHub's compare commits API to compare the commits between head of the branch being released, and the found sha of the previous release, allowing the previous release's sha to be on a different branch. This allows, for example, to have a branch like release-1.7.x, with the latest release being, say, 1.7.4, and then release a new minor version (1.8.0) from a branch like release-1.8.x, and have it correctly show the changelog of commits since 1.7.4, even though that tag is on a different branch.

This PR isn't complete yet, and there's a lot of test coverage missing, but I wanted to get consensus on the approach before investing more time. Thanks!

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