vim-patch:8.1.0375: cannot use diff mode with Cygwin diff.exe

Problem:    Cannot use diff mode with Cygwin diff.exe. (Igor Forca)
Solution:   Skip over unrecognized lines in the diff output.

3b8defd0a5

src/nvim/testdir/test_diffmode.vim

@@ -517,7 +517,9 @@ func Test_diffexpr()
   endif
 
   func DiffExpr()
-    silent exe '!diff ' . v:fname_in . ' ' . v:fname_new . '>' . v:fname_out
+    " Prepend some text to check diff type detection
+    call writefile(['warning', '  message'], v:fname_out)
+    silent exe '!diff ' . v:fname_in . ' ' . v:fname_new . '>>' . v:fname_out
   endfunc
   set diffexpr=DiffExpr()
   set diffopt=foldcolumn:0