feat: for new branches, automatically create a remote branch on first push

[dwilding]

Currently, gimmegit doesn't create a remote branch when you create a local branch. So git push doesn't work:

$ git push
fatal: The current branch snapshot0830 has no upstream branch.
To push the current branch and set the remote as upstream, use

    git push --set-upstream origin snapshot0830

To have this happen automatically for branches without a tracking
upstream, see 'push.autoSetupRemote' in 'git help config'.

This PR configures cloned repos so that on first push:

• A remote branch is created (push.default = current)
• The remote branch is set as the tracking branch (push.autoSetupRemote = true)

Here's what git push looks like with this config:

$ git push
Enumerating objects: 4, done.
Counting objects: 100% (4/4), done.
Delta compression using up to 14 threads
Compressing objects: 100% (2/2), done.
Writing objects: 100% (3/3), 971 bytes | 971.00 KiB/s, done.
Total 3 (delta 1), reused 0 (delta 0), pack-reused 0
remote: Resolving deltas: 100% (1/1), completed with 1 local object.
remote: 
remote: Create a pull request for 'snapshot0830' on GitHub by visiting:
remote:      https://github.com/dwilding/jubilant/pull/new/snapshot0830
remote: 
To github.com:dwilding/jubilant.git
 * [new branch]      snapshot0830 -> snapshot0830
branch 'snapshot0830' set up to track 'origin/snapshot0830'.