Hide git diff for t/*.data files

While technically ASCII text files, git makes the questionable decision
to show text diffs on t/*.data files. But these diffs are very big and
noisy, which can slow down some tooling to a crawl, and they make little
sense to a human reader anyway because it’s mostly just binary encoded
in Base64.

This commit instructs Git to treat these data files as if they were
binary. From the gitattributes(5) man page:

> Git usually guesses correctly whether a blob contains text or binary
> data by examining the beginning of the contents. However, sometimes
> you may want to override its decision, either because a blob contains
> binary data later in the file, or because the content, while
> technically composed of text characters, is opaque to a human reader.
> For example, many postscript files contain only ASCII characters, but
> produce noisy and meaningless diffs.

Author: marc-vanderwal

.gitattributes

@@ -0,0 +1,2 @@
+# While technically text, diffs are meaningless on test data files.
+t/*.data -diff