feat(git-node): use a single git push command

[aduh95]

I cannot see why we would prefer having several git push commands. However, I see a number of reasons to have only one, at the very end of the --promote sequence:

• it makes it less likely a release would go half through, e.g. if the releaser has to abandon the release before the git push for whatever reason.
• for folks watching the repo, the delay between "the proposal branch gets merged" and "the release tag is out" can be frustrating.
• it removes the need for a --continue (cc @RafaelGSS); with this PR, you either have pushed and the only remaining command is tools/release.sh, or you haven't pushed and can start over.

[RafaelGSS]

LGTM

[aduh95]

It's probably wiser to wait for a release proposal PR to be open so this can be tested, I reckon I might be missing a .trim() call somewhere

[RafaelGSS]

Right, so I'm going to land #868 without this change