Standardize some of the scopes used.
Add “fixup!” snippet for commit messages.

In a Git commit message typing ‘fix⇥’ will show a menu with the last 15 commit subjects.

This is useful when using ‘git rebase -i …’ with auto-squash enabled (see ‘rebase.autosquash’ in git help config).
Add scoping of ‘fixup!’ and ‘squash!’ keywords.
Prevent unstaged changed files from being highlighted.
Overhaul grammar to use new features.

- Correct fixup/squash! match.
- Use meta.embedded scope.
- Alter the portion of the line matched for highlighting changed files.
- Limit altered files match to only committed files.
Move to using \z, simplify long line matches.
Change shortcut to ⌃⇧S and add attr.scm.git scope.
Disable spell checking inside commit metadata.
Add scope and change shortcut.
Remove shortcut and add class for new SCM bundle.
Add require for SCM bundle.
Better end changed files while before diff block.
Remove command to update bundle.
Start using TM_SCM_COMMIT_WINDOW.
Scope and classify these commands.
Don't count fixup|squash in line length.
Don't match empty lines with foldingStopMarker.
Add semantic class so this appears in the menu.
Disable softWrap for diffs inside commit messages.
Cleanup with recent changes.