build: add a script to merge a PR according to our current workflow

[coriolinus]

We have decided to require both signed commits and a linear history in github, which breaks the github "merge" button completely.

This script does the right thing to accomplish signed commits with a linear history, because it's kind of a pain to do manually each time.

What's new in this PR

---

PR Submission Checklist for internal contributors

• The PR Title
  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • contains a reference JIRA issue number like SQPIT-764
  • answers the question: If merged, this PR will: ... ³

1. https://sparkbox.com/foundry/semantic_commit_messages
2. https://github.com/wireapp/.github#usage
3. E.g. feat(conversation-list): Sort conversations by most emojis in the title #SQPIT-764.

[github-actions]

Bencher Report

Branch	prgn/build/merge-script
Testbed	ubuntu-latest

Click to view all benchmark results

Benchmark	Latency	milliseconds (ms)
Commit add f(group size)/cs1/mem/1002	📈 view plot 🚷 view threshold	18.70
Commit add f(group size)/cs1/mem/2	📈 view plot 🚷 view threshold	6.83
Commit add f(group size)/cs1/mem/202	📈 view plot 🚷 view threshold	9.40
Commit add f(group size)/cs1/mem/402	📈 view plot 🚷 view threshold	11.88
Commit add f(group size)/cs1/mem/602	📈 view plot 🚷 view threshold	15.14
Commit add f(group size)/cs1/mem/802	📈 view plot 🚷 view threshold	16.92
Commit add f(number clients)/cs1/mem/1002	📈 view plot 🚷 view threshold	989.30
Commit add f(number clients)/cs1/mem/2	📈 view plot 🚷 view threshold	7.03
Commit add f(number clients)/cs1/mem/202	📈 view plot 🚷 view threshold	84.54
Commit add f(number clients)/cs1/mem/402	📈 view plot 🚷 view threshold	220.08
Commit add f(number clients)/cs1/mem/602	📈 view plot 🚷 view threshold	426.49
Commit add f(number clients)/cs1/mem/802	📈 view plot 🚷 view threshold	676.43
Commit pending proposals f(group size)/cs1/mem/1002	📈 view plot 🚷 view threshold	117.76
Commit pending proposals f(group size)/cs1/mem/2	📈 view plot 🚷 view threshold	28.41
Commit pending proposals f(group size)/cs1/mem/202	📈 view plot 🚷 view threshold	46.43
Commit pending proposals f(group size)/cs1/mem/402	📈 view plot 🚷 view threshold	61.50
Commit pending proposals f(group size)/cs1/mem/602	📈 view plot 🚷 view threshold	79.89
Commit pending proposals f(group size)/cs1/mem/802	📈 view plot 🚷 view threshold	95.87
Commit pending proposals f(pending size)/cs1/mem/1	📈 view plot 🚷 view threshold	19.92
Commit pending proposals f(pending size)/cs1/mem/101	📈 view plot 🚷 view threshold	115.47
Commit pending proposals f(pending size)/cs1/mem/21	📈 view plot 🚷 view threshold	35.73
Commit pending proposals f(pending size)/cs1/mem/41	📈 view plot 🚷 view threshold	56.88
Commit pending proposals f(pending size)/cs1/mem/61	📈 view plot 🚷 view threshold	76.25
Commit pending proposals f(pending size)/cs1/mem/81	📈 view plot 🚷 view threshold	100.90
Commit remove f(group size)/cs1/mem/1002	📈 view plot 🚷 view threshold	27.57
Commit remove f(group size)/cs1/mem/2	📈 view plot 🚷 view threshold	6.65
Commit remove f(group size)/cs1/mem/202	📈 view plot 🚷 view threshold	8.86
Commit remove f(group size)/cs1/mem/402	📈 view plot 🚷 view threshold	11.87
Commit remove f(group size)/cs1/mem/602	📈 view plot 🚷 view threshold	17.06
Commit remove f(group size)/cs1/mem/802	📈 view plot 🚷 view threshold	21.78
Commit remove f(number clients)/cs1/mem/1002	📈 view plot 🚷 view threshold	29.85
Commit remove f(number clients)/cs1/mem/2	📈 view plot 🚷 view threshold	137.14
Commit remove f(number clients)/cs1/mem/202	📈 view plot 🚷 view threshold	115.11
Commit remove f(number clients)/cs1/mem/402	📈 view plot 🚷 view threshold	93.71
Commit remove f(number clients)/cs1/mem/602	📈 view plot 🚷 view threshold	71.71
Commit remove f(number clients)/cs1/mem/802	📈 view plot 🚷 view threshold	50.42
Commit update f(group size)/cs1/mem/1002	📈 view plot 🚷 view threshold	136.83
Commit update f(group size)/cs1/mem/2	📈 view plot 🚷 view threshold	6.95
Commit update f(group size)/cs1/mem/202	📈 view plot 🚷 view threshold	33.48
Commit update f(group size)/cs1/mem/402	📈 view plot 🚷 view threshold	59.56
Commit update f(group size)/cs1/mem/602	📈 view plot 🚷 view threshold	86.45
Commit update f(group size)/cs1/mem/802	📈 view plot 🚷 view threshold	111.31

🐰 View full continuous benchmarking report in Bencher

[istankovic]

I think this is a prime use case for git merge-base ---is-ancestor. 😄

[coriolinus]

Haha, I was looking for something like that, but google wasn't helpful. I think this behavior, which does the rebase as a side-effect, is actually better though.

There are 3 cases we care about:

1. The branch already originates from origin/main
2. The branch originates from some older commit than origin/main but applies cleanly.
3. The branch originates from some older commit than origin/main but does not apply cleanly.

git merge-base --is-ancestor will do a great job distinguishing case 1, but conflates cases 2 and 3. We do want to distinguish between those.

git rebase as written here conflates 1 and 2, but distinguishes case 3; I think that's the behavior we actually want.

[istankovic]

Looks good, with a few smaller things.