fix: handle git scissor line in commit message parsing

[oberwager]

Description

Fixes commit message validation failure when using git commit --verbose. The hook incorrectly validated diff content that appears after Git's scissor line (# ------------------------ >8 ------------------------), failing when diff lines exceeded the body length limit.

Changes:

• Modified read_commit_message() to detect and strip content after the scissor line
• Added test coverage for this scenario

Rationale: Git's scissor line behavior is documented to ignore everything after this marker, but the linter was processing the entire file including verbose output.

Related

• Git commit documentation on verbose mode
• Git scissor line behavior in Git source tests

Testing

• All existing tests pass (65/65)
• Added specific test case with scissor line + diff content
• Manual testing with git commit --verbose confirms fix
• No breaking changes to existing functionality

[github-actions]

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