Use binary merge strategy for code generator hashes.

The binary merge strategy marks the file as conflicted but doesn't
actually change the file contents. This keeps the json hashes file
valid and alerts the user that the generator needs to be re-run.

Bug: angleproject:3083
Change-Id: Ia04e96d56a2d92ee061f25e7de79b6a1fd3ce4ab
Reviewed-on: https://chromium-review.googlesource.com/c/1423519
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

Author: null77

.gitattributes

@@ -8,3 +8,7 @@
 **/shaders/gen/*.inc eol=lf
 *.sh                 eol=lf
 *.gn                 eol=lf
+
+# Git conflict markers in the json file break the code generator.
+# Using a binary merge strategy forces a conflict without changing the file.
+scripts/run_code_generation_hashes.json merge=binary