Default to existing branch names when generating stacks

src/stack_pr/cli.py

@@ -631,18 +631,78 @@ def get_next_available_branch_name(branch_name_template: str, name: str) -> str:
     return generate_branch_name(branch_name_template, int(branch_id or 0) + 1)
 
 
+def get_commit_branch_name(commit_id: str) -> str | None:
+    """Get the branch name that a commit is currently on.
+
+    Args:
+        commit_id: The commit hash to check.
+
+    Returns:
+        The branch name if the commit is on a branch, None otherwise.
+    """
+    # Get all branches that contain this commit
+    branches = get_command_output(
+        ["git", "branch", "--points-at", commit_id]
+    ).splitlines()
+
+    # Clean up branch names by removing leading spaces and asterisks
+    branches = [branch.lstrip(" *") for branch in branches]
+    # Filter out HEAD references
+    branches = [
+        branch
+        for branch in branches
+        if branch != "HEAD" and not branch.startswith("(HEAD")
+    ]
+
+    if not branches:
+        return None
+
+    # If the commit is on multiple branches, prefer the current branch
+    current_branch = get_current_branch_name()
+    if current_branch in branches:
+        return current_branch
+
+    return branches[0]
+
+
 def set_head_branches(
-    st: list[StackEntry], remote: str, *, verbose: bool, branch_name_template: str
+    stack: list[StackEntry],
+    remote: str,
+    *,
+    verbose: bool,
+    branch_name_template: str,
 ) -> None:
-    """Set the head ref for each stack entry if it doesn't already have one."""
+    """Set the head ref for each stack entry using the branch name the commit is currently on.
+
+    If a commit is not on any branch, it will use the default branch name template.
 
+    Args:
+        stack: List of stack entries to process.
+        remote: Name of the remote repository.
+        verbose: Whether to print verbose output.
+        branch_name_template: Template string for generating branch names.
+    """
     run_shell_command(["git", "fetch", "--prune", remote], quiet=not verbose)
     available_name = get_available_branch_name(remote, branch_name_template)
-    for e in filter(lambda e: not e.has_head(), st):
-        e.head = available_name
-        available_name = get_next_available_branch_name(
-            branch_name_template, available_name
-        )
+
+    for entry in filter(lambda e: not e.has_head(), stack):
+        # Try to get the branch name the commit is currently on
+        branch_name = get_commit_branch_name(entry.commit.commit_id())
+        if branch_name:
+            # If the commit is on a branch, use that branch name
+            entry.head = branch_name
+        else:
+            # If the commit is not on any branch, fall back to the default template
+            entry.head = available_name
+            available_name = get_next_available_branch_name(
+                branch_name_template, available_name
+            )
+
+        if verbose:
+            log(
+                f"Using branch name '{entry.head}' for commit {entry.commit.commit_id()[:8]}",
+                level=2,
+            )
 
 
 def init_local_branches(