fix: use git cli for diff

[didroe]

Description

Replace go-git with the git CLI. We've seen some issues with the diff output from go-git which is causing diff scans to include unchanged findings.

Also centralises the lookup of Git metadata and uses the flags/options to gather all the inputs.

Testing

• Regular:
  • Not a git repository
  • No commits
  • No remote
• Diff:
  • DIFF_BASE_BRANCH implies --diff (for backwards compatibility)
  • Not a git repository
  • No commits
  • No remote
  • Uncommitted changes
  • Custom merge base commit
  • Detached
  • Github
  • Gitlab
• SaaS:
  • Not a git repository
  • No remote
  • No commits
  • Detached

Related

• Close Base branch findings are attributed to the wrong head lines #1396
• Close Bearer scan on uncommitted files #1397 (Approach (1) taken where we compute a different cache key if some uncommitted changes are detected)

Checklist

• I've added test coverage that shows my fix or feature works as expected.
• I've updated or added documentation if required.
• I've included usage information in the description if CLI behavior was updated or added.
• PR title follows Conventional Commits format