fix(remote): resolve HEAD to current branch for remote metadata

[leno23]

Summary

• When the commit range ends with HEAD (e.g. v1.0.0..HEAD from --unreleased), remote integrations now resolve HEAD to the current local branch name before calling GitHub/GitLab/etc. APIs
• Previously HEAD was mapped to None, which made APIs fall back to the repository default branch and miss PR metadata for commits on feature branches
• Uses git2 via Repository::head_ref_name() instead of spawning git rev-parse

Fixes #1274

Supersedes the approach in draft PR #1275 with a git2-based implementation aligned with existing repository handling.

Test plan

• cargo test -p git-cliff-core repo::test::head_ref_name_returns_current_branch

[codecov-commenter]

Codecov Report

❌ Patch coverage is 63.63636% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.66%. Comparing base (7e35ff5) to head (58801a4).

Files with missing lines	Patch %	Lines
git-cliff-core/src/changelog.rs	58.83%	7 Missing ⚠️
git-cliff-core/src/repo.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1535      +/-   ##
==========================================
+ Coverage   48.63%   48.66%   +0.03%     
==========================================
  Files          26       26              
  Lines        2287     2300      +13     
==========================================
+ Hits         1112     1119       +7     
- Misses       1175     1181       +6     

Flag	Coverage Δ
unit-tests	48.66% <63.64%> (+0.03%)	⬆️

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