PR landing tool

[lundibundi]

Basically, a command line tool that will automatically pick up commits from the branch add metadata to them and merge into the specified branch.

• cherry-pick commits from source branch
• automatically add PR-URL to the commit message if it's not present (potentially automatically determining it from the GitHub API)
• support fixup! and squash! commits
• rebasing source branch if needed and requested
• allowing to optionally verify\augment each commit message as they are picked up
• support squash-all that must call squash command on all commits (except first) and prompt (git prompt) user for the resulting commit message
• allow to automatically comment Landed in ... in the PR.

Edit:
The tool should probably store/restore some kind of state from the fs to be able to support manual editing of commit messages and perhaps fixing of merge conflicts, as it may require to stop the tool and interact with the git itself.

/cc @belochub @nechaido

[JuliaGerasymenko]

@lundibundi Which objectives do you have for each task? One of them is how should I manage with getting information?