test-diff-color: disable pager for expected output on Windows (issue5555)

Windows uses `more.com`, which unhelpfully adds an extra trailing line
consisting only of '\r'.  It also converts tab characters to spaces, which
throws off the last two tests.

Setting the 'ui.formatted' option is what allowed the pager to be used by these
tests in the first place.

tests/test-diff-color.t

@@ -4,6 +4,7 b' Setup'
   > [ui]
   > color = yes
   > formatted = always
+  > paginate = never
   > [color]
   > mode = ansi
   > EOF