git-combine: synthesize a monorepo

Created by: keegancsmith

This introduces a new command "git-combine" which will combine all upstream HEAD branches into a single repository. For each commit upstream it will create a new commit with the file contents as a sub directory (the remote name).

Additionally this comes with a naive "daemon" mode which just runs git fetch --all, git-combine, git push on a timer. This is running in production right now and the result can be seen at https://github.com/sgtest/megarepo

Fixes https://github.com/sourcegraph/sourcegraph/issues/27492