feat: Add squash merge support to git-merge-pr promotion step

[bobdoah]

Adds a new 'mergeMethod' configuration option to the git-merge-pr promotion step that allows users to specify the merge strategy when merging pull requests.

This fulfils the feature request I opened in #5580.

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[hiddeco]

Thanks for implementing this, and I think it is a fair feature request. I made a couple of suggestions here and there.

My primary concern, other than what's commented, is that we silently ignore merge methods not supported by the selected provider. I think we should either:

1. Return an error when a user specifies a merge method that the provider doesn't support
2. Log a warning so users understand their preference wasn't honored

I'd lean toward option 1, as it's typically better to fail explicitly than to silently do something different from what the user requested.

[codecov]

Codecov Report

❌ Patch coverage is 70.32967% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.34%. Comparing base (9c066ee) to head (cfb87c2).
⚠️ Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/gitprovider/azure/azure.go	34.48%	19 Missing ⚠️
pkg/gitprovider/bitbucket/bitbucket.go	28.57%	5 Missing ⚠️
pkg/gitprovider/gitea/gitea.go	90.47%	2 Missing ⚠️
pkg/gitprovider/gitprovider.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5581      +/-   ##
==========================================
+ Coverage   56.29%   56.34%   +0.04%     
==========================================
  Files         426      426              
  Lines       32262    32308      +46     
==========================================
+ Hits        18163    18204      +41     
- Misses      13045    13050       +5     
  Partials     1054     1054              

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[bobdoah]

Thanks for implementing this, and I think it is a fair feature request. I made a couple of suggestions here and there.

My primary concern, other than what's commented, is that we silently ignore merge methods not supported by the selected provider. I think we should either:

1. Return an error when a user specifies a merge method that the provider doesn't support
2. Log a warning so users understand their preference wasn't honored

I'd lean toward option 1, as it's typically better to fail explicitly than to silently do something different from what the user requested.

Hey, you're welcome and thanks for the comments.

I had considered either of those two options. I didn't see how I might go about option 2. I guess I thought it would fail how it currently does. I can see how option 1 is preferable, and help users.

Hopefully my latest changes address your comments. Happy to change/reword anything if you've got any other preferences!

[bobdoah]

I've made some further changes as the code coverage drop was flagged as a failure.

[hiddeco]

I think this is best combined with #5454 as I suspect the resolved SHA for GitLab will otherwise be wrong.