_git: Remove TODOs of actually documented options
_git: correct some "undocumented" options

diff options:
  --cumulative is deprecated --> remove
  --follow was already in there --> remove duplicate
  -G / -S: reword -S to emphase the difference to -G

revision options:
  --abbrev: reword, to emphase the --abbrev-commit requirement
_git: Change completion text for consistency
_git: Move --(no-)follow to be log-specific option