fix: don't be tripped up by empty stacks in the vb.toml adapter.

We can't handle them well there and it's unclear what to do,
and we generally ignore them

Author: Byron

crates/but-graph/src/ref_metadata_legacy.rs

@@ -323,7 +323,7 @@ impl RefMetadata for VirtualBranchesTomlMetadata {
                     }) {
                         Some(branch) => branch,
                         None => branches_to_create.pop().context(
-                            "BUG: do not pop off the last branch, remove the whole stack",
+                            "BUG: incoming stack is probably empty, caller should have removed the whole stack",
                         )?,
                     };
 
@@ -560,6 +560,8 @@ impl VirtualBranchesTomlMetadata {
             ref_info: managed_ref_info(),
             stacks: stacks
                 .iter()
+                // We aren't able to handle these well, so let's ignore them.
+                .filter(|stack| !stack.heads.is_empty())
                 .sorted_by_key(|s| s.order)
                 .map(|s| WorkspaceStack {
                     id: s.id,

crates/but-graph/tests/graph/ref_metadata_legacy.rs

@@ -1054,7 +1054,7 @@ fn create_workspace_from_scratch_workspace_first() -> anyhow::Result<()> {
     let err = store.set_workspace(&ws).unwrap_err();
     assert_eq!(
         err.to_string(),
-        "BUG: do not pop off the last branch, remove the whole stack"
+        "BUG: incoming stack is probably empty, caller should have removed the whole stack"
     );
     ws.stacks.pop();
     assert_eq!(ws.stacks.len(), 1);