Detect when the parent is the same as current branch

We encounter sometime loops in stacky when the parent is the same as
branch and we are caught in an infinite loop.

Author: mpatou

src/stacky/stacky.py

@@ -261,10 +261,14 @@ def get_real_stack_bottom() -> Optional[BranchName]:  # type: ignore [return]
         return candiates.pop()
 
 
+def get_stack_parent_branch_name(branch: BranchName) -> str:
+    return "branch.{}.merge".format(branch)
+
+
 def get_stack_parent_branch(branch: BranchName) -> Optional[BranchName]:  # type: ignore [return]
     if branch in STACK_BOTTOMS:
         return None
-    p = run(CmdArgs(["git", "config", "branch.{}.merge".format(branch)]), check=False)
+    p = run(CmdArgs(["git", "config", get_stack_parent_branch_name(branch)]), check=False)
     if p is not None:
         p = remove_prefix(p, "refs/heads/")
         return BranchName(p)
@@ -447,6 +451,12 @@ def load_stack_for_given_branch(
             if check:
                 die("Branch is not in a stack: {}", branch)
             return None, [b.branch for b in branches]
+        if branch == parent:
+            error("Branch {} seems to be its own parent, this is wrong", branch)
+            error(
+                f"To fix it update the config for {get_stack_parent_branch_name(branch)} by running git config {get_stack_parent_branch_name(branch)} ref/heads/<branch>"
+            )
+            die("Refusing to continue")
         branch = parent
 
     branches.append(BranchNCommit(branch, None))
@@ -1154,7 +1164,7 @@ def set_parent(branch: BranchName, target: BranchName, *, set_origin: bool = Fal
             [
                 "git",
                 "config",
-                "branch.{}.merge".format(branch),
+                get_stack_parent_branch_name(branch),
                 "refs/heads/{}".format(target),
             ]
         )