.gitattributes: Mark minified emscripten js files as -diff

This prevents the contents of these files showing up in the
output of `git grep`.  This makes git grep much more useful as
these files are minified into a single line which is not human
readable.

Change-Id: I54047fe32d090570fa70935ce108455a47e4d888
Reviewed-on: https://chromium-review.googlesource.com/1232674
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Sam Clegg <sbc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56049}

.gitattributes

@@ -3,3 +3,5 @@
 # Do not modify line endings for binary files (which are sometimes auto
 # detected as text files by git).
 *.png binary
+# Don't include minified JS in git grep/diff output
+test/mjsunit/asm/sqlite3/*.js -diff