Fix the test failures related to remote default branch (HEAD) — heartwood

The following two tests assumed that HEAD would always be there in the ‘rad’ remote. It turns out, however, that this isn’t necessarily the case, for example on Debian releases (as consistently noticed in multiple CI runs).

This change fixes this issue by explicitly setting the remote head, where this matters:

crates/radicle-cli/examples/rad-patch-fetch-2.md
crates/radicle-cli/examples/rad-patch-via-push.md

 $ git pull
 Already up to date.
 $ git branch -r
   rad/HEAD -> rad/master
   rad/master
   rad/patches/5e2dedcc5d515fcbc1cca483d3376609fe889bfb
 ```

 $ git show-ref
 d894a83c9c356552a57af09ccdbd5587a99045 refs/heads/feature/1
 f2de534b5e81d7c6e2dcaf58c3dd91573c0a0354 refs/heads/master
 f2de534b5e81d7c6e2dcaf58c3dd91573c0a0354 refs/remotes/rad/HEAD
 f2de534b5e81d7c6e2dcaf58c3dd91573c0a0354 refs/remotes/rad/master
 d894a83c9c356552a57af09ccdbd5587a99045 refs/remotes/rad/patches/6035d2f582afbe01ff23ea87528ae523d76875b6
 ```

         let mut config = repo.config().unwrap();
         config.set_str("user.name", USER_NAME).unwrap();
         config.set_str("user.email", USER_EMAIL).unwrap();
         // Git older than v2.48.0 have the behaviour of 'remote.<remote>.followRemoteHEAD=never',
         // So we set it explicitly here for testing purposes, for consistent output of commands
         // like 'git branch -r' and 'git show-ref'
         config
             .set_str("remote.rad.followRemoteHEAD", "never")
             .unwrap();
     }
     let sig = git::raw::Signature::new(