tests: make test independent of git setting of default branch

[radoering]

Summary by Sourcery

Ensure git backend tests use a repository with a fixed 'main' default branch and update related expectations to be independent of external git default-branch settings.

Tests:

• Initialize temporary git test repositories with 'main' as the default branch to decouple tests from environment-specific git settings.
• Adjust clone error expectation to reference 'refs/heads/main' instead of 'refs/heads/master' in git backend tests.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

This PR makes the Git-based tests independent of the environment’s default branch by explicitly initializing test repositories with 'main' as the default branch and updating the corresponding expected error message to reference 'refs/heads/main' instead of 'refs/heads/master'.

File-Level Changes

Change	Details	Files
Ensure temporary dulwich test repositories use a deterministic default branch name.	Initialize the temporary dulwich repository with an explicit default_branch argument set to b'main' instead of relying on dulwich’s default. Keep the rest of the temp_repo fixture behavior (worktree retrieval and commit setup) unchanged.	tests/vcs/git/conftest.py
Align test expectations with the new default branch name used during cloning failures.	Update the expected error message in the clone-existing-locked-tag test to assert against 'refs/heads/main' instead of 'refs/heads/master'. Keep the rest of the test logic and assertions unchanged.	tests/vcs/git/test_backend.py

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[sourcery-ai]

Hey there - I've reviewed your changes - here's some feedback:

• To truly decouple the test from a specific default branch name, consider deriving the expected ref from the created repo (e.g., using repo.default_branch or a fixture parameter) instead of hardcoding 'main' in the error message assertion.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- To truly decouple the test from a specific default branch name, consider deriving the expected ref from the created repo (e.g., using `repo.default_branch` or a fixture parameter) instead of hardcoding `'main'` in the error message assertion.

## Individual Comments

### Comment 1
<location> `tests/vcs/git/test_backend.py:279` </location>
<code_context>

     expected_short = (
-        f"Failed to clone {source_url} at 'refs/heads/master',"
+        f"Failed to clone {source_url} at 'refs/heads/main',"
         f" unable to acquire file lock for {tag_ref}."
     )
</code_context>

<issue_to_address>
**suggestion (testing):** The assertion on the exact error string is quite brittle; consider relaxing it to focus on the important parts.

`assert str(exc_info.value) == expected_short` ties the test to the exact wording (including punctuation and spacing), so minor message refactors will break it even if behavior is unchanged. Prefer asserting on key substrings (e.g., branch ref and lock path) or using a regex so the test verifies `refs/heads/main` and the tag lock are mentioned without over-constraining the full message.
</issue_to_address>

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[radoering]

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