Pull request conflicts: Support merge=union in .gitattributes file

[rtzoeller]

This is a continuation of isaacs/github#487, as I could not find an existing one.

https://github.com/nix-rust/nix has recently seen a number of PRs which only conflict on a CHANGELOG.md file. This forces PRs to frequently need to be manually rebased. The changelog is marked as merge=union in a .gitattributes file so these rebases are trivial to perform locally, but it can make accepting contributions from external contributors challenging.

It would be helpful if GitHub supported this attribute natively, so manual rebases were not required.

[pscott31]

Please GitHub :)

[JTHuang-Nunu]

hope this

[matthiasgoergens]

Looks like github is using a bad re-implementation of half of git internally?

[kit-ty-kate]

*taps the mic* is anyone even working on the core product at Github anymore?

[fboja]

+1 for getting at least partial support for merge=union being respected from .gitattributes .

We have a whole team running into merge conflicts on PRs daily due this issue.

We can avoid the conflicts by doing client-side branch merging (as that respects .gitattributes merge rules), but that would require relaxing branch protection rules to allow commits directly on target branches which would be a major downgrade for keeping our codebase safe.

TL;DR: GitHub PRs ignoring .gitattributes forces us to manually having to work around basic Git functionality missing from Github.