More defensively build the format string in case the workspace commit isn't managed (#10466)

Note that it's still unclear how the graph workspace can report that it's not pointed to
by the managed workspace ref, yet the first commit it finds seems to be the managed
workspace commit.

This really shouldn't be possible.

Author: Byron

crates/but-workspace/src/lib.rs

@@ -221,8 +221,8 @@ pub struct RefInfo {
 #[derive(Debug, Clone)]
 pub struct AncestorWorkspaceCommit {
     /// The commits along the first parent that are between the managed workspace reference and the managed workspace commit.
-    /// The vec is never empty.
-    pub commits_outside: Vec<crate::ref_info::Commit>,
+    /// The vec *should* not be empty, but it can be empty in practice for reasons yet to be discovered.
+    pub commits_outside: Vec<ref_info::Commit>,
     /// The index of the segment that actually holds the managed workspace commit.
     pub segment_with_managed_commit: SegmentIndex,
     /// The index of the workspace commit within the `commits` array in its parent segment.

crates/but-workspace/src/ref_info.rs

@@ -337,7 +337,7 @@ pub(crate) mod function {
     use crate::ref_info::{LocalCommit, LocalCommitRelation};
     use crate::ui::PushStatus;
     use crate::{AncestorWorkspaceCommit, RefInfo, WorkspaceCommit, branch};
-    use anyhow::{Context, bail};
+    use anyhow::bail;
     use but_core::ref_metadata::ValueInfo;
     use but_graph::petgraph::Direction;
     use but_graph::{
@@ -488,10 +488,17 @@ pub(crate) mod function {
             msg.push_str(
                     "Run the following command in your working directory to fix this and restore the committed changes.\n\n",
                 );
-            msg.push_str(&format!("    git stash && git reset --hard {ws_commit_id} && git checkout {user_commit_id} -- .",
-                                      user_commit_id = info.commits_outside
-                                                              .first()
-                                                              .context("BUG: at least one user commit on top")?.id));
+            msg.push_str(&format!(
+                "    git stash && git reset --hard {ws_commit_id}{checkout_clause}",
+                checkout_clause = info
+                    .commits_outside
+                    .first()
+                    .map(|c| format!(
+                        " && git checkout {user_commit_id} -- .",
+                        user_commit_id = c.id
+                    ))
+                    .unwrap_or_default()
+            ));
             bail!("{msg}");
         }
         info.compute_similarity(graph, repo, opts.expensive_commit_info)?;