Auto merge of #134199 - RalfJung:gitmerge, r=oli-obk

Revert "Stop git from merging generated files"

This reverts rust-lang/rust#133851. "-merge" makes git not even do merges if they are entirely conflict-free, which is not the behavior we want. We sometimes have conflict-free merges in generated files and it's much better if git can handle them automatically.

r? `@oli-obk`
Cc `@jieyouxu` `@Urgau`

Author: bors