Add feature_category to Gitlab::Git spec files

What does this MR do and why?

Adds feature_category: :source_code_management to 20 spec files in spec/lib/gitlab/git/ that were missing this metadata.

This aligns with other specs in the same directory (which already use source_code_management) and reduces the RSpec/FeatureCategory rubocop todo list by 20 entries.

Files updated

File	Description
spec/lib/gitlab/git/attributes_at_ref_parser_spec.rb	Git attributes parsing at ref
spec/lib/gitlab/git/base_error_spec.rb	Git base error handling
spec/lib/gitlab/git/blob_spec.rb	Git blob operations
spec/lib/gitlab/git/bundle_file_spec.rb	Git bundle file validation
spec/lib/gitlab/git/changes_spec.rb	Git changes tracking
spec/lib/gitlab/git/commit_stats_spec.rb	Commit statistics
spec/lib/gitlab/git/conflict/parser_spec.rb	Merge conflict parsing
spec/lib/gitlab/git/conflict/resolver_spec.rb	Merge conflict resolution
spec/lib/gitlab/git/cross_repo_spec.rb	Cross-repository operations
spec/lib/gitlab/git/gitmodules_parser_spec.rb	Submodule parsing
spec/lib/gitlab/git/lfs_changes_spec.rb	LFS changes detection
spec/lib/gitlab/git/lfs_pointer_file_spec.rb	LFS pointer files
spec/lib/gitlab/git/merge_base_spec.rb	Merge base calculation
spec/lib/gitlab/git/patches/collection_spec.rb	Patch collections
spec/lib/gitlab/git/patches/patch_spec.rb	Individual patches
spec/lib/gitlab/git/pre_receive_error_spec.rb	Pre-receive hook errors
spec/lib/gitlab/git/raw_diff_change_spec.rb	Raw diff changes
spec/lib/gitlab/git/remote_mirror_spec.rb	Remote mirroring
spec/lib/gitlab/git/util_spec.rb	Git utilities
spec/lib/gitlab/git/wiki_page_version_spec.rb	Wiki page versioning

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR adds feature_category metadata only - no functional changes
• All specs pass rubocop RSpec/FeatureCategory check
• Feature category matches other specs in the same directory