patch 8.0.1704: 'backupskip' default doesn't work for Mac

Problem:    'backupskip' default doesn't work for Mac.
Solution:   Use "/private/tmp". (Rainer Müller, closes #2793)

src/option.c

@@ -3367,7 +3367,11 @@ set_init_1(int clean_arg)
 	    mustfree = FALSE;
 # ifdef UNIX
 	    if (*names[n] == NUL)
+#  ifdef MACOS_X
+		p = (char_u *)"/private/tmp";
+#  else
 		p = (char_u *)"/tmp";
+#  endif
 	    else
 # endif
 		p = vim_getenv((char_u *)names[n], &mustfree);

src/testdir/test_options.vim

@@ -332,3 +332,19 @@ func Test_set_indentexpr()
   call assert_equal('', &indentexpr)
   bwipe!
 endfunc
+
+func Test_backupskip()
+  if has("mac")
+    call assert_match('/private/tmp/\*', &bsk)
+  elseif has("unix")
+    call assert_match('/tmp/\*', &bsk)
+  endif
+
+  let bskvalue = substitute(&bsk, '\\', '/', 'g')
+  for var in  ['$TEMPDIR', '$TMP', '$TEMP']
+    if exists(var)
+      let varvalue = substitute(expand(var), '\\', '/', 'g')
+      call assert_match(varvalue . '.\*', bskvalue)
+    endif
+  endfor
+endfunc

src/version.c

@@ -762,6 +762,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1704,
 /**/
     1703,
 /**/