Update gitattributes so that objc diffs are correctly contextualized.

anntzer · anntzer · commit 3d95af15f633 · 2021-11-26T00:11:10.000+01:00
i.e., when showing a diff on _macosx.m, the `@@ ...` header shows the
correct current function.
diff --git a/.gitattributes b/.gitattributes
@@ -1,4 +1,5 @@
 * text=auto
+*.m diff=objc
 *.ppm binary
 *.svg binary
 *.svg linguist-language=true
