Automatically infer a remote branch as a base for a stack without a local copy of it

[epage]

Please complete the following tasks

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Version

v0.8.0

Use Case

Some users would rather not have a local main branch. See #93.

We need to keep in mind the following cases

• A repo with a long history of release branches
• A repo with every developer's topic branches

For a sense of scale, gecko-dev has 600+ remote branches

Requirements

• Works like today without the need for a local main
• Little negative performance impact on repos like gecko-dev
• Don't bloat the stack view with hundreds of "empty stacks"

Possible Solutions

• We could enumerate all of them and treat them as protected branches. That would bloat the UI and probably slow things down as we try to determine which feature branch belongs to which protected branch.
• We could hide protected branches that have no commits but there are valid cases for that (e.g. when you've wrapped up all feature branches, your base disappears) and there is still the potential for performance issues
• We could add a ignored-branch setting, like protected-branch, but we've been aiming for working out of the gate as much as possible but this could force people to configure git-stack just to use it