Add ability to pull other branches' release logs automatically

[thanethomson]

In our workflow in CometBFT, we'll often ready a changelog for a particular release on that release branch (e.g. on the v0.38.x branch), and then once we cut the release (e.g. v0.38.0) we need to pull those changelog entries into main, because that's where most people landing in the repo would find the most up-to-date changelog.

Instead of doing this manually, it'd be good to be able to "pull" any missing changelog entries into one's current branch from any other branch.

Suggested workflow after cutting a release from a particular release branch (e.g. v0.38.x):

# First stash/commit any local changes and pull remote changes to all release branches

# Pull entries from the given branch into the current branch
> unclog pull v0.38.x

# Pull entries from multiple branches in one go
> unclog pull v0.38.x v0.37.x

[mpoke]

@thanethomson Adding this feature would be really great. I guess it will combine very nicely with removing duplicates, i.e., #81.