_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
Change completion text for consistency
Move --(no-)follow to be git-log-specific option