remote-helper: allow patch creation from detached HEAD

The patch creation flow would be partially interrupted when creating a
patch via a detached HEAD state. The creation of an upstream branch
would fail since there is no branch to set an upstream for. It would
result in the following error:

    ✓ Patch 6035d2f582afbe01ff23ea87528ae523d76875b6 opened
    To rad://z42hL2jL4XNk6K8oHQaSWfMgCL7ji/z6MknSLrJoTcukLrE435hVNQT4JUhbvWLX4kUzqkEStBU8Vi
     ! [remote rejected] HEAD -> refs/patches (git: reference 'HEAD' is neither a local nor a remote branch.; class=Invalid (3))
    error: failed to push some refs to 'rad://z42hL2jL4XNk6K8oHQaSWfMgCL7ji/z6MknSLrJoTcukLrE435hVNQT4JUhbvWLX4kUzqkEStBU8Vi'

To fix this, the reference that's resolved is checked to see if it a
local branch. If not, it will return early.

The behaviour is tested in the `rad-patch-detached-head` example.

Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>
X-Clacks-Overhead: GNU Terry Pratchett

radicle-cli/examples/rad-patch-detached-head.md

@@ -0,0 +1,41 @@
+To ensure that we can handle diverse workflows, we also allow patches to be
+opened when we're in the infamous 'detached HEAD' state.
+
+First, we will enter this state by using `git checkout` on a commit object:
+
+``` (stderr) RAD_HINT=1
+$ git checkout f2de534b5e81d7c6e2dcaf58c3dd91573c0a0354
+Note: switching to 'f2de534b5e81d7c6e2dcaf58c3dd91573c0a0354'.
+
+You are in 'detached HEAD' state. You can look around, make experimental
+changes and commit them, and you can discard any commits you make in this
+state without impacting any branches by switching back to a branch.
+
+If you want to create a new branch to retain commits you create, you may
+do so (now or later) by using -c with the switch command. Example:
+
+  git switch -c <new-branch-name>
+
+Or undo this operation with:
+
+  git switch -
+
+Turn off this advice by setting config variable advice.detachedHead to false
+
+HEAD is now at f2de534 Second commit
+```
+
+Now, we can create a commit on top of this and create a patch, as usual:
+
+``` (stderr) RAD_HINT=1
+$ git commit -a -m "Add things" -q --allow-empty
+$ git push -o patch.message="Add things #1" -o patch.message="See commits for details." rad HEAD:refs/patches
+✓ Patch 6035d2f582afbe01ff23ea87528ae523d76875b6 opened
+hint: offline push, your node is not running
+hint: to sync with the network, run `rad node start`
+To rad://z42hL2jL4XNk6K8oHQaSWfMgCL7ji/z6MknSLrJoTcukLrE435hVNQT4JUhbvWLX4kUzqkEStBU8Vi
+ * [new reference]   HEAD -> refs/patches
+```
+
+Note that there will be no upstream branch, since we did not have a branch to
+set an upstream for in the first place!

radicle-cli/tests/commands.rs

@@ -1044,6 +1044,26 @@ fn rad_patch_via_push() {
     .unwrap();
 }
 
+#[test]
+fn rad_patch_detached_head() {
+    let mut environment = Environment::new();
+    let profile = environment.profile(config::profile("alice"));
+    let working = tempfile::tempdir().unwrap();
+    let home = &profile.home;
+
+    // Setup a test repository.
+    fixtures::repository(working.path());
+
+    test("examples/rad-init.md", working.path(), Some(home), []).unwrap();
+    test(
+        "examples/rad-patch-detached-head.md",
+        working.path(),
+        Some(home),
+        [],
+    )
+    .unwrap();
+}
+
 #[test]
 fn rad_patch_merge_draft() {
     let mut environment = Environment::new();

radicle/src/rad.rs

@@ -353,6 +353,11 @@ pub fn setup_patch_upstream<'a>(
         return Ok(None);
     };
 
+    // Can't set an upstream for something that's not a branch
+    if !r.is_branch() {
+        return Ok(None);
+    }
+
     let branch = git::raw::Branch::wrap(r);
 
     // Only set the upstream if it's missing or `force` is `true`