fix: parse file paths with spaces in diff header

[ColonelBucket8]

Fixes #537

• Fix cannot unstage file with spaces
• Add test case for file with spaces for stages and unstages

Checklist

• The modified code has some test-coverage (where applicable).
• make test is passing (this is what CI runs).
• New unreleased features/fixes/styling and performance improvements are documented via git: feat: / fix: / style: / perf:. Or e.g. perf(highlighting):.
  See https://github.com/altsem/gitu/blob/master/docs/dev-tooling.md