refactor(dag): extract query_stack_commits to Dag

So that we can use this logic in `git-branchless-submit`, which wants to be fundamentally aware of commit stacks as part of submitting.

Author: arxanas

git-branchless-lib/src/core/dag.rs

@@ -527,6 +527,30 @@ impl Dag {
         })
     }
 
+    /// Determine the connected components among draft commits (commit "stacks")
+    /// that intersect with the provided set.
+    #[instrument]
+    pub fn query_stack_commits(&self, commit_set: CommitSet) -> eyre::Result<CommitSet> {
+        let draft_commits = self.query_draft_commits()?;
+        let stack_roots = self.query_roots(draft_commits.clone())?;
+        let stack_ancestors = self.query_range(stack_roots, commit_set)?;
+        let stack = self
+            // Note that for a graph like
+            //
+            // ```
+            // O
+            // |
+            // o A
+            // | \
+            // |  o B
+            // |
+            // @ C
+            // ```
+            // this will return `{A, B, C}`, not just `{A, C}`.
+            .query_range(stack_ancestors, draft_commits.clone())?;
+        Ok(stack)
+    }
+
     /// Wrapper around DAG method.
     #[instrument]
     pub fn query_all(&self) -> eyre::Result<CommitSet> {

git-branchless-revset/src/builtins.rs

@@ -290,28 +290,9 @@ fn fn_draft(ctx: &mut Context, name: &str, args: &[Expr]) -> EvalResult {
 #[instrument]
 fn fn_stack(ctx: &mut Context, name: &str, args: &[Expr]) -> EvalResult {
     let arg = eval0_or_1(ctx, name, args)?.unwrap_or_else(|| ctx.dag.head_commit.clone());
-    let draft_commits = ctx
-        .dag
-        .query_draft_commits()
-        .map_err(EvalError::OtherError)?;
-    let stack_roots = ctx.dag.query_roots(draft_commits.clone())?;
-    let stack_ancestors = ctx.dag.query_range(stack_roots, arg)?;
-    let stack = ctx
-        .dag
-        // Note that for a graph like
-        //
-        // ```
-        // O
-        // |
-        // o A
-        // | \
-        // |  o B
-        // |
-        // @ C
-        // ```
-        // this will return `{A, B, C}`, not just `{A, C}`.
-        .query_range(stack_ancestors, draft_commits.clone())?;
-    Ok(stack)
+    ctx.dag
+        .query_stack_commits(arg)
+        .map_err(EvalError::OtherError)
 }
 
 type MatcherFn = dyn Fn(&Repo, &Commit) -> Result<bool, PatternError> + Sync + Send;