feat: Support --diff3 flag in update command

[ytausch]

Background

We want to use mergiraf with copier update in an automated copier update workflow. Mergiraf cannot be directly configured as a merge driver for our use case because copier update uses git merge-file internally. Hence, we rely on executing mergiraf in interactive mode after copier update has been run. We need diff3-style conflict markers for this. git merge-file cannot be globally configured to generate diff3-styled conflict markers; hence copier needs to pass the --diff3 flag to it.

I am aware of #2376, but this PR implements a quicker solution for immediate use with mergiraf.

Summary

• Add --diff3 flag to the copier update command that produces diff3-style conflict markers including the base version (from last update)
• This makes conflict resolution easier by showing what the original content was before both local and template changes
• Particularly useful for semantic merge tools like mergiraf

AI Note

I generated this PR with assistance from Claude Code, but reviewed it manually.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.34%. Comparing base (9bed140) to head (6916d22).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2488      +/-   ##
==========================================
+ Coverage   97.33%   97.34%   +0.01%     
==========================================
  Files          55       55              
  Lines        6405     6432      +27     
==========================================
+ Hits         6234     6261      +27     
  Misses        171      171              

Flag	Coverage Δ
unittests	97.34% <100.00%> (+0.01%)	⬆️

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[ytausch]

xref https://lore.kernel.org/git/48B1AA62-D7FF-439E-B770-1127E1EE0E79@ytausch.de/T/#u

[ytausch]

Setting to draft because in the thread linked above, the core maintainer of git (Junio C Hamano) replied that they are willing to accept a patch that would enable the git merge-file behaviour to be configured globally. This would make this PR redundant.