previous commit gets amended

[gc23]

Once switched to 4.2.0 (from 4.0.0) whenever I run standard-version the last commit gets amended and the new commit by standard-version is added.

The part that is amending the commit should not happen. That commit ends up having a new commit id, which in turn causes the local branch to diverge from the remote.

Easy way to reproduce is to :

1. Create a new commit
2. Push to remote
3. run standard-version
4. git status will give out Your branch and 'origin/master' have diverged

Tested on MacOs node 8.1.2

[bcoe]

@stevemao any thoughts on this one? I believe we only git add and git commit don't know how we'd end up diverged.