x/review/git-codereview: add merge command

[FiloSottile]

Manual merge commits are dangerous (#26199, and it seems to have happened before with a merge from release-branch.go1.10).

We use merges for two reasons:

1. to merge master into another branch to keep it up to date
2. to merge dev branches into master at the end of their cycle

The first case is much more frequent, while the second case is what we don't want to happen most of the times, so the tooling should only support the first.

[mvdan]

Perhaps it would also be useful to restrict who can push directly to master? Or perhaps pushes to it should require a "yes I really am trying to push master" confirmation or flag.

[FiloSottile]

Pushing to master is already strictly limited (and I plan to make it opt in also for the people who can). This mistaken merge happened via a regular Gerrit review.

[aclements]

For #43147, our attempted merge actually wound up like a cherry-pick of all changes from master on to dev.regabi, with just a single parent (so it wasn't even a merge commit). We believe this is what happened:

$ git change dev.regabi
$ git merge master
# Fix merge conflicts, add files
$ git commit
git-codereview: cannot commit on dev.regabi branch
$ git merge --continue
git-codereview: cannot commit on dev.regabi branch
$ git change m
$ git mail

We believe the git change m at the end got confused by the in-progress uncommitted merge state and wound up creating a cherry-pick-like commit on branch m with that state.

At a minimum, git change X should refuse to do anything if you're in a pending merge state.

Maybe even allowing the git change dev.regabi was a problem, since it wasn't going to allow commits on that. Perhaps it should either just allow committing to that name (personally I find it really annoying that it restricts what branch names you can use), or it should disallow changing to names that can't be committed to. The latter would probably require a flag to git change to say "make branch X that's for upstream branch Y", which may be a good thing to have anyway.

[gopherbot]

Change https://golang.org/cl/279772 mentions this issue: git-codereview: new sync-branch and related fixes