fix(record): correct handling of --stash with detached head

This addresses an overlooked review comment from #1150:

> issue: Won't this not work if no branch is checked out before trying to
> stash? In that case, it seems like git record --stash would make the commit
> without switching back to the parent commit.

And, indeed, that's exactly the case!

Ref: #1150 (comment)

Author: claytonrcarter

git-branchless-record/src/lib.rs

@@ -221,9 +221,19 @@ fn record(
             ResolvedReferenceInfo {
                 oid: Some(oid),
                 reference_name: _,
-            } => Some(CheckoutTarget::Oid(oid)),
+            } => {
+                let head_commit = repo.find_commit_or_fail(oid)?;
+                match head_commit.get_parents().as_slice() {
+                    [] => None,
+                    [parent_commit] => Some(CheckoutTarget::Oid(parent_commit.get_oid())),
+                    parent_commits => {
+                        eyre::bail!("git-branchless record --stash seems to have created a merge commit, but this should be impossible. Parents: {parent_commits:?}");
+                    }
+                }
+            }
             _ => None,
         };
+
         if stash && checkout_target.is_some() {
             try_exit_code!(check_out_commit(
                 effects,

git-branchless-record/tests/test_record.rs

@@ -342,6 +342,53 @@ fn test_record_stash() -> eyre::Result<()> {
     Ok(())
 }
 
+#[test]
+fn test_record_stash_detached_head() -> eyre::Result<()> {
+    let git = make_git()?;
+
+    if !git.supports_reference_transactions()? {
+        return Ok(());
+    }
+    git.init_repo()?;
+
+    git.detach_head()?;
+
+    {
+        git.commit_file("test1", 1)?;
+
+        let stdout = git.smartlog()?;
+        insta::assert_snapshot!(stdout, @r###"
+        O f777ecc (master) create initial.txt
+        |
+        @ 62fc20d create test1.txt
+        "###);
+    }
+
+    {
+        git.write_file_txt("test1", "new test1 contents\n")?;
+
+        let (stdout, _stderr) = git.branchless("record", &["-m", "foo", "--stash"])?;
+        insta::assert_snapshot!(stdout, @r###"
+        [detached HEAD 9b6164c] foo
+         1 file changed, 1 insertion(+), 1 deletion(-)
+        branchless: running command: <git-executable> checkout 62fc20d2a290daea0d52bdc2ed2ad4be6491010e
+        "###);
+    }
+
+    {
+        let stdout = git.smartlog()?;
+        insta::assert_snapshot!(stdout, @r###"
+        O f777ecc (master) create initial.txt
+        |
+        @ 62fc20d create test1.txt
+        |
+        o 9b6164c foo
+        "###);
+    }
+
+    Ok(())
+}
+
 #[test]
 fn test_record_stash_default_message() -> eyre::Result<()> {
     let git = make_git()?;