auto git add / commit when running dvc repro

[AlJohri]

would be great to automatically git add/git commit after dvc repro. this may in the future enable something like dvc repro --all-pipelines --all-branches

ref discord convo: https://discordapp.com/channels/485586884165107732/485596304961962003/542085672941518929

[efiop]

Related #1558

[jorgeorpinel]

Maybe if dvc install has been run ("full Git integration mode").

[efiop]

We now have auto-stage option #4543 . Doesn't look like there is a lot of demand for comitting part.