fix(bump): use commit subject for non-conventional bumps

[jagath-sajjan]

Description

Fixes version bump detection for multiline custom commits when conventional_commits = false.

Previously, git-cliff passed the full non-conventional commit message into version bump detection. When a multiline commit was provided via --with-commit, custom_major_increment_regex and custom_minor_increment_regex could be ignored, causing an incorrect patch bump.

This change makes nonconventional commits use only the subject line (the first line) for bump calculation, while preserving the full message for changelog generation. It also adds regression tests covering multiline major and minor bump cases.

Motivation and Context

Fixes #1476.

This is needed for users who disable conventional commits and rely on custom prefixes like breaking or feat at the start of free-form commit subjects. With this fix, multiline --with-commit input behaves consistently with single-line input for version bumping.

How Has This Been Tested?

Added regression coverage in git-cliff-core/src/release.rs for:

• Multiline non-conventional commits matching custom_major_increment_regex
• Multiline non-conventional commits matching custom_minor_increment_regex

Commands run:

• cargo fmt --all
• cargo test -p git-cliff-core release::test::bump_version -- --nocapture

Notes:

• The targeted regression test passed.
• cargo test -p git-cliff-core still showed existing unrelated failures in repo::test::{get_latest_tag, git_tags, resolves_existing_tag_with_name_and_message} in my local environment.

Screenshots / Logs (if applicable)

N/A

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly (if applicable).
• I have formatted the code with rustfmt.
  • cargo +nightly fmt --all
• I checked the lints with clippy.
  • cargo clippy --tests --verbose -- -D warnings
• I have added tests to cover my changes.
• All new and existing tests passed.
  • cargo test

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.69%. Comparing base (39cd76e) to head (47ab8c7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1478      +/-   ##
==========================================
+ Coverage   47.35%   47.69%   +0.34%     
==========================================
  Files          24       24              
  Lines        2131     2135       +4     
==========================================
+ Hits         1009     1018       +9     
+ Misses       1122     1117       -5     

Flag	Coverage Δ
unit-tests	47.69% <100.00%> (+0.34%)	⬆️

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