Mark the definitive place for the new Unmerged stack relation

It's left for later to implement, if we want to use it.
It will definitely be relevant for worktrees, so let's not forget about it.

Author: Byron

crates/but-workspace/src/commit.rs

@@ -37,15 +37,15 @@ impl std::fmt::Debug for Stack {
 
 /// Structures related to creating a merge-commit along with the respective tree.
 pub mod merge {
+    use super::Stack;
+    use crate::WorkspaceCommit;
     use anyhow::{Context, bail};
+    use but_core::ref_metadata::WorkspaceCommitRelation;
     use but_graph::SegmentIndex;
     use gitbutler_oxidize::GixRepositoryExt;
     use gix::prelude::ObjectIdExt;
     use tracing::instrument;
 
-    use super::Stack;
-    use crate::WorkspaceCommit;
-
     /// A minimal stack for to represent a stack that conflicted.
     #[derive(Debug, Clone)]
     pub struct ConflictingStack {
@@ -125,8 +125,17 @@ pub mod merge {
             let mut missing_stacks = Vec::new();
             let mut tips: Vec<_> = stacks
                 .into_iter()
-                .filter_map(|s| s.branches.first())
-                .filter_map(|top_segment| {
+                .filter_map(|s| s.branches.first().map(|b| (b, s.workspacecommit_relation)))
+                .filter_map(|(top_segment, relation)| {
+                    match relation {
+                        WorkspaceCommitRelation::Merged => {}
+                        WorkspaceCommitRelation::UnmergedTree => {
+                            // These need to be part of the parents list, but shouldn't be merged.
+                            // If the caller wants to retry them, they can be passed here as "Merged".
+                            todo!("this is a placeholder for where we will have to start handling this UnmergedTree")
+                        }
+                        WorkspaceCommitRelation::Outside => return None,
+                    }
                     let stack_tip_name = top_segment.ref_name.as_ref();
                     match graph.segment_and_commit_by_ref_name(stack_tip_name) {
                         None => {