fix: add --extra-files flag to stage additional files in bump commit

[maimul]

Problem

There is no way to include generated files (changelogs, build artifacts, lockfiles) in the version bump commit when those files are not part of any [bumpversion:file:...] section. Users are forced to make a separate commit or use --allow-dirty.

Fix

Add --extra-files FILE [FILE ...] CLI flag (also settable as extra_files = ... in config) that stages arbitrary additional files alongside the bump commit. Files can be dirty or untracked.

Changes

cli.py

• extra_files added to the list-value parsing loop in _load_configuration
• --extra-files argument added to _parse_arguments_phase_2
• _determine_vcs_dirty accepts extra_files=None and passes them to assert_nondirty as allowed_paths so the dirty check doesn't abort when extra files are modified
• _commit_to_vcs accepts extra_files=None and calls vcs.add_extra_path() for each before committing

vcs.py

• Git.assert_nondirty accepts allowed_paths=None; paths in that set are excluded from the dirty check
• Git.add_extra_path uses git add <path> (without --update) so untracked files can be staged
• BaseVCS.add_extra_path is a no-op so Mercurial doesn't break

Usage

bumpversion patch --extra-files docs/changelog.md dist/summary.txt

Or in config:

[bumpversion]
extra_files = docs/changelog.md
              dist/summary.txt

Fixes #259