Revert "Stop git from merging generated files"

This reverts commit 4cbb599.
"-merge" makes git not even do merges if they are entirely conflict-free,
which is not the behavior we want.

Author: RalfJung

.gitattributes

@@ -4,10 +4,8 @@
 *.cpp rust
 *.h rust
 *.rs rust diff=rust
-*.fixed linguist-language=Rust -merge
-*.mir linguist-language=Rust -merge
-*.stderr -merge
-*.stdout -merge
+*.fixed linguist-language=Rust
+*.mir linguist-language=Rust
 src/etc/installer/gfx/* binary
 src/vendor/** -text
 Cargo.lock linguist-generated=false