patch 8.1.2182: test42 seen as binary by git diff

Problem:    Test42 seen as binary by git diff.
Solution:   Add .gitattributes file.  Make explicit that 'cpo' does not
            contain 'S'. (Daniel Hahler, closes #5072)
diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000..9216dbe
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1 @@
+src/testdir/test42.in diff