update verify logic and test case for merge-undo (#273)

# Exercise Review

## Exercise Discussion
#269 

Removed the unnecessary check that enforces students to be on `main`
branch when running `verify`.
## Checklist

- [ ] If you require a new remote repository on the `Git-Mastery`
organization, have you [created a
request](https://github.com/git-mastery/exercises/issues/new?template=request_exercise_repository.yaml)
for it?
- [ ] Have you written unit tests using
[`repo-smith`](https://github.com/git-mastery/repo-smith) to validate
the exercise grading scheme?
- [ ] Have you tested your changes using the instructions posted?
- [ ] Have you verified that this exercise does not already exist or is
not currently in review?
- [ ] Did you introduce a new grading mechanism that should belong to
[`git-autograder`](https://github.com/git-mastery/git-autograder)?
- [ ] Did you introduce a new dependency that should belong to
[`app`](https://github.com/git-mastery/app)?

Author: SAN-MUYUN

merge_undo/test_verify.py

@@ -5,8 +5,8 @@
 from .verify import (
     MAIN_WRONG_COMMIT,
     MERGES_NOT_UNDONE,
-    NOT_ON_MAIN,
     RESET_MESSAGE,
+    MAIN_BRANCH_MISSING,
     verify,
 )
 
@@ -105,7 +105,7 @@ def test_main_wrong_commit():
         )
 
 
-def test_not_main():
+def test_main_branch_missing():
     with loader.start() as (test, rs):
         _create_and_commit_file(rs, "rick.txt", "Scientist", "Add Rick")
         _create_and_commit_file(rs, "morty.txt", "Boy", "Add Morty")
@@ -127,7 +127,7 @@ def test_not_main():
             """,
             "Mention Morty is grandson",
         )
-        rs.git.checkout("daughter")
+        rs.git.branch("master", old_branch="main", move=True)
 
         output = test.run()
-        assert_output(output, GitAutograderStatus.UNSUCCESSFUL, [NOT_ON_MAIN])
+        assert_output(output, GitAutograderStatus.UNSUCCESSFUL, [MAIN_BRANCH_MISSING])

merge_undo/verify.py

@@ -4,26 +4,21 @@
     GitAutograderStatus,
 )
 
-NOT_ON_MAIN = (
-    "You aren't currently on the main branch. Checkout to that branch and try again!"
+MAIN_BRANCH_MISSING = (
+    "Main branch is missing, you can reset the exercises if needed and try again."
 )
-DETACHED_HEAD = "You should not be in a detached HEAD state! Use git checkout main to get back to main"
 MERGES_NOT_UNDONE = "It appears the merge commits are still in the history of the 'main' branch. This shouldn't be the case"
 MAIN_WRONG_COMMIT = "The 'main' branch is not pointing to the correct commit. It should be pointing to the commit made just before the merges."
 RESET_MESSAGE = 'Reset the repository using "gitmastery progress reset" and start again'
 SUCCESS_MESSAGE = "Great work with undoing the merges! Try listing the directory to see what has changed."
 
 
 def verify(exercise: GitAutograderExercise) -> GitAutograderOutput:
-    repo = exercise.repo.repo
+    main_branch = exercise.repo.branches.branch_or_none("main")
 
-    try:
-        if repo.active_branch.name != "main":
-            raise exercise.wrong_answer([NOT_ON_MAIN])
-    except TypeError:
-        raise exercise.wrong_answer([DETACHED_HEAD, RESET_MESSAGE])
+    if not main_branch:
+        raise exercise.wrong_answer([MAIN_BRANCH_MISSING, RESET_MESSAGE])
 
-    main_branch = exercise.repo.branches.branch("main")
     main_history = main_branch.commits
 
     if any(len(c.commit.parents) > 1 for c in main_history):