Force Git to treat testorig.ppm as a binary file

Otherwise, because the file begins with an ASCII header, Git will
erroneously treat is as an ASCII file, and if Git for Windows is
configured with default options (specifically, "Checkout windows-style,
commit Unix-style line endings"), it will add carriage return characters
to all of the "linefeed" characters in the PPM file, thus corrupting it
and causing libjpeg-turbo's regression tests to fail.

.gitattributes

@@ -3,3 +3,4 @@
 /ci export-ignore
 /.gitattributes export-ignore
 /.github export-ignore
+*.ppm binary