GitInput: Determine default branch for repository

[mweinelt]

Up until now when no branch was defined the Git Plugin hardcoded the master branch.

Now we try resolving the default branch with git ls-remote and allow a configurable fallback branch, that can be master or anything else.

Tested!

Closes: #325

[mweinelt]

starting evaluation of jobset ‘nixos-mailserver:.jobsets (jobset#43)’ (last checked 39 s ago)
(nixos-mailserver:.jobsets) Fetching input `decl` (git) https://gitlab.com/simple-nixos-mailserver/nixos-mailserver
checking out Git branch main from https://gitlab.com/simple-nixos-mailserver/nixos-mailserver
Initialized empty Git repository in /tmp/git-checkout-tmp-pvfWa3Qd/source/.git/
From /var/lib/hydra/scm/git/e772881f93683bcbbf09c2f3678b8c329ac0aa377ec18540e20cf8cdf013a6e6
 * branch            main       -> FETCH_HEAD
 * [new branch]      main       -> origin/main
Switched to a new branch 'fetchgit'
removing `.git'...
hash is 20059ba4f6ebd93275667aa2db2d6452fd183762713f060a57ad55b330af5a9a
path is /nix/store/2vdfq8d0awib7sqqbya0vmmcfdcimdic-source

Tested and successfully pulls main (the default branch on nixos-mailserver) without any branch specified or any specific configuration.