git-repo: Add git network support back to signer

[jku]

The plan in #31 includes making both repository tools and signer integrate with git enough that

• GH actions become simpler in "production"
• the tools can be run in a local git repo for testing purposes
• signer hopefully becomes easier to use

currently signer flow is

1. get signing event branch (either create or pull from remote)
2. run playground-sign
3. git add & git commit
4. git push to remote (that may or may not be same as the pull-remote)

I'd like this to be embedded in playground-sign:

• "push-remote" and "pull-remote" need to be configured in .playground-sign.ini
• signing-event (branch) name is either given as CLI arg (in this case fetch from remote, use a temp local branch)., Otherwise maybe we can make up the name?
  • it would be cool if playground-sign could figure out any currently open signing-events without a name...
• git commit we can handle
• git push to signing-event branch on push-remote -- this needs to be togglable with --push like in the repo tools