Only commit package.json instead of all files

[fmiras]

Is it viable to just add package.json after bumping version instead making a git add -A? I can send the PR myself

https://github.com/zeit/release/blob/0b421e071e9bcbf66bbfe4702a0743145260ac1e/lib/bump.js#L112

cc @leo

[leo]

Sounds like a great idea!