Add experimental mergify configuration

[AlekSi]

Description

See https://mergify.com and https://docs.mergify.com/configuration/

That configuration should remove the need to constantly click the "Update branch" button.

It probably will be replaced by a GitHub-native merge queue (https://github.com/features/merge-queue/signup) when we get access to it.

Readiness checklist

• I added tests for new functionality or bugfixes.
• I ran task all, and it passed.
• I added/updated comments for both exported and unexported top-level declarations (functions, types, etc).
• I checked comments rendering with task godocs.
• I ensured that the title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[vercel]

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Name	Status	Preview	Updated
ferret-db	✅ Ready (Inspect)	Visit Preview	Oct 19, 2022 at 8:59AM (UTC)

[codecov]

Codecov Report

Merging #1281 (7223b54) into main (0c29ff7) will decrease coverage by 0.02%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1281      +/-   ##
==========================================
- Coverage   67.12%   67.10%   -0.03%     
==========================================
  Files         270      270              
  Lines       12722    12722              
==========================================
- Hits         8540     8537       -3     
- Misses       3275     3277       +2     
- Partials      907      908       +1     

Impacted Files	Coverage Δ
internal/util/version/version.go	48.52% <0.00%> (-4.42%)	⬇️
internal/handlers/common/filter.go	86.58% <0.00%> (ø)

Flag	Coverage Δ
integration	63.01% <ø> (-0.03%)	⬇️
mongodb	14.09% <ø> (-0.01%)	⬇️
pg	52.42% <ø> (-0.03%)	⬇️
tigris	34.06% <ø> (-0.03%)	⬇️
unit	25.05% <ø> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[rumyantseva]

The configuration looks strange to me - are the conditions "or" conditions or "and" conditions? It's not straightforward from the docs, but if you checked it, I'm ok with the configuration.

[AlekSi]

It is "and" by default:

If a pull request matches all of the listed conditions in a rule, the subsequent actions listed in that rule will be applied.

[AlekSi]

@rumyantseva PTAL

[rumyantseva]

I'd prefer a longer syntax (not and and being mentioned explicitly) to improve readability and not confuse people, but it's not a strong opinion, so I'm ready to approve it.

[AlekSi]

All their examples use implicit and: https://docs.mergify.com/examples/ Just like MongoDB with filters without $and :)

[chilagrow]

Let's try it out!