[PATCH] Fix bug in diffopt_changed() function

[Jason Franklin]

Greetings,

While reading this article...

  https://vimways.org/2018/the-power-of-diff/

... yesterday, I discovered a subtle bug in the behavior when
the 'diffopt' setting is changed.

To observe, use the two attached files and run...

  vim --clean
  :e f2.rb
  :vsp f1.rb
  :windo diffthis

Now, you can source the following two snippets of VimL:

  :set diffopt&
  :set diffopt+=algorithm:patience,indent-heuristic

Observe the correct behavior after the above.

  :set diffopt&
  :set diffopt+=indent-heuristic,algorithm:patience

Observe the incorrect behavior.  This is because order matters
in the "diffopt_changed()" function.

I believe the patch, also attached, fixes this problem.  It'd be
great if Christian could take a look and verify my change.

I didn't include a test because I thought I'd be more testing the
output of the diff operation than the setting of the option.  That is,
there's no way to get direct access to the internal data managed by
the option when set.  Let me know if this is not acceptable.

Best,
Jason Franklin

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f1.rb
Description: Binary data

f2.rb
Description: Binary data

diff --git a/src/diff.c b/src/diff.c
index 0399e7967..7f7e15df6 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -2173,6 +2173,7 @@ diffopt_changed(void)
     int		diff_flags_new = 0;
     int		diff_foldcolumn_new = 2;
     long	diff_algorithm_new = 0;
+    long	diff_indent_heuristic = 0;
     tabpage_T	*tp;
 
     p = p_dip;
@@ -2236,7 +2237,7 @@ diffopt_changed(void)
 	else if (STRNCMP(p, "indent-heuristic", 16) == 0)
 	{
 	    p += 16;
-	    diff_algorithm_new |= XDF_INDENT_HEURISTIC;
+	    diff_indent_heuristic = XDF_INDENT_HEURISTIC;
 	}
 	else if (STRNCMP(p, "internal", 8) == 0)
 	{
@@ -2276,6 +2277,8 @@ diffopt_changed(void)
 	    ++p;
     }
 
+    diff_algorithm_new |= diff_indent_heuristic;
+
     /* Can't have both "horizontal" and "vertical". */
     if ((diff_flags_new & DIFF_HORIZONTAL) && (diff_flags_new & DIFF_VERTICAL))
 	return FAIL;