git: don't show diffs of files in .aspect/

These files don't contain info that's interesting to most, so let's
remove them from diffs.

Epic: none
Release note: None

.gitattributes

@@ -1,2 +1,3 @@
+.aspect/** -diff
 build/bazelutil/distdir_files.bzl -diff
 pkg/BUILD.bazel -diff