patch 8.1.0513: no error for set diffopt+=algorithm:

Problem:    No error for set diffopt+=algorithm:.
Solution:   Check for missing argument. (Hirohito Higashi, closes #3598)

src/diff.c

@@ -2266,6 +2266,8 @@ diffopt_changed(void)
 		p += 9;
 		diff_algorithm_new = XDF_HISTOGRAM_DIFF;
 	    }
+	    else
+		return FAIL;
 	}
 
 	if (*p != ',' && *p != NUL)

src/testdir/gen_opt_test.vim

@@ -81,7 +81,7 @@ let test_values = {
       \ 'cryptmethod': [['', 'zip'], ['xxx']],
       \ 'cscopequickfix': [['', 's-', 's-,c+,e0'], ['xxx', 's,g,d']],
       \ 'debug': [['', 'msg', 'msg', 'beep'], ['xxx']],
-      \ 'diffopt': [['', 'filler', 'icase,iwhite'], ['xxx']],
+      \ 'diffopt': [['', 'filler', 'icase,iwhite'], ['xxx', 'algorithm:xxx', 'algorithm:']],
       \ 'display': [['', 'lastline', 'lastline,uhex'], ['xxx']],
       \ 'eadirection': [['', 'both', 'ver'], ['xxx', 'ver,hor']],
       \ 'encoding': [['latin1'], ['xxx', '']],

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    513,
 /**/
     512,
 /**/