Fix commands.c build issue on merge (redis#10172)

Set commands.c's merge driver to binary so when it conflicts during a merge git will leave the local version unmodified.
This way our Makefile will rebuild it based on src/commands/*.json before trying to compile it.
Otherwise the file gets modified with merge conflict markers and gets the same timestamp as the *.json files,
so the Makefile doesn't attempt to rebuild it before compiling and we get a compilation error.

Author: yoav-steinberg

.gitattributes

@@ -0,0 +1,6 @@
+# We set commands.c's merge driver to `binary` so when it conflicts during a
+# merge git will leave the local version unmodified. This way our Makefile
+# will rebuild it based on src/commands/*.json before trying to compile it.
+# Otherwise the file gets modified and gets the same timestamp as the .json
+# files. So the Makefile doesn't attempt to rebuild it before compiling.
+src/commands.c merge=binary