vim-patch:8.2.2490: 'wrap' option is always reset when starting diff mode

Problem:    'wrap' option is always reset when starting diff mode.
Solution:   Add the "followwrap" item in 'diffopt'. (Rick Howe, closes vim/vim#7797)
https://github.com/vim/vim/commit/4223d43c0fb6ead1e611e4469a1680a9228b6015

4 files changed, 32 insertions, 10 deletions

diff --git a/runtime/doc/diff.txt b/runtime/doc/diff.txt
index 2a972483ff..3544882ceb 100644
--- a/runtime/doc/diff.txt
+++ b/runtime/doc/diff.txt
@@ -48,7 +48,7 @@ In each of the edited files these options are set:
 	'scrollbind'	on
 	'cursorbind'	on
 	'scrollopt'	includes "hor"
-	'wrap'		off
+	'wrap'		off, or leave as-is if 'diffopt' includes "followwrap"
 	'foldmethod'	"diff"
 	'foldcolumn'	value from 'diffopt', default is 2
 
@@ -132,7 +132,7 @@ Otherwise they are set to their default value:
 	'scrollbind'	off
 	'cursorbind'	off
 	'scrollopt'	without "hor"
-	'wrap'		on
+	'wrap'		on, or leave as-is if 'diffopt' includes "followwrap"
 	'foldmethod'	"manual"
 	'foldcolumn'	0
 
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 98e2d50f1d..ce3788e0fc 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -1971,6 +1971,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 		foldcolumn:{n}	Set the 'foldcolumn' option to {n} when
 				starting diff mode.  Without this 2 is used.
 
+		followwrap	Follow the 'wrap' option and leave as it is.
+
 		internal	Use the internal diff library.  This is
 				ignored when 'diffexpr' is set.  *E960*
 				When running out of memory when writing a
diff --git a/src/nvim/diff.c b/src/nvim/diff.c
index 358725239c..31b7b1bd8f 100644
--- a/src/nvim/diff.c
+++ b/src/nvim/diff.c
@@ -58,6 +58,7 @@ static bool diff_need_update = false;  // ex_diffupdate needs to be called
 #define DIFF_HIDDEN_OFF 0x100   // diffoff when hidden
 #define DIFF_INTERNAL   0x200   // use internal xdiff algorithm
 #define DIFF_CLOSE_OFF  0x400   // diffoff when closing window
+#define DIFF_FOLLOWWRAP 0x800   // follow the wrap option
 #define ALL_WHITE_DIFF (DIFF_IWHITE | DIFF_IWHITEALL | DIFF_IWHITEEOL)
 static int diff_flags = DIFF_INTERNAL | DIFF_FILLER | DIFF_CLOSE_OFF;
 
@@ -1361,11 +1362,12 @@ void diff_win_options(win_T *wp, int addbuf)
     wp->w_p_crb_save = wp->w_p_crb;
   }
   wp->w_p_crb = true;
-
-  if (!wp->w_p_diff) {
-    wp->w_p_wrap_save = wp->w_p_wrap;
+  if (!(diff_flags & DIFF_FOLLOWWRAP)) {
+    if (!wp->w_p_diff) {
+      wp->w_p_wrap_save = wp->w_p_wrap;
+    }
+    wp->w_p_wrap = false;
   }
-  wp->w_p_wrap = false;
   curwin = wp;  // -V519
   curbuf = curwin->w_buffer;
 
@@ -1437,11 +1439,11 @@ void ex_diffoff(exarg_T *eap)
         if (wp->w_p_crb) {
           wp->w_p_crb = wp->w_p_crb_save;
         }
-
-        if (!wp->w_p_wrap) {
-          wp->w_p_wrap = wp->w_p_wrap_save;
+        if (!(diff_flags & DIFF_FOLLOWWRAP)) {
+          if (!wp->w_p_wrap) {
+            wp->w_p_wrap = wp->w_p_wrap_save;
+          }
         }
-
         free_string_option(wp->w_p_fdm);
         wp->w_p_fdm = vim_strsave(*wp->w_p_fdm_save
                                   ? wp->w_p_fdm_save
@@ -2158,6 +2160,9 @@ int diffopt_changed(void)
     } else if (STRNCMP(p, "closeoff", 8) == 0) {
       p += 8;
       diff_flags_new |= DIFF_CLOSE_OFF;
+    } else if (STRNCMP(p, "followwrap", 10) == 0) {
+      p += 10;
+      diff_flags_new |= DIFF_FOLLOWWRAP;
     } else if (STRNCMP(p, "indent-heuristic", 16) == 0) {
       p += 16;
       diff_indent_heuristic = XDF_INDENT_HEURISTIC;
diff --git a/src/nvim/testdir/test_diffmode.vim b/src/nvim/testdir/test_diffmode.vim
index f09a64c329..640de1bdc6 100644
--- a/src/nvim/testdir/test_diffmode.vim
+++ b/src/nvim/testdir/test_diffmode.vim
@@ -964,6 +964,21 @@ func Test_diff_closeoff()
   enew!
 endfunc
 
+func Test_diff_followwrap()
+  new
+  set diffopt+=followwrap
+  set wrap
+  diffthis
+  call assert_equal(1, &wrap)
+  diffoff
+  set nowrap
+  diffthis
+  call assert_equal(0, &wrap)
+  diffoff
+  set diffopt&
+  bwipe!
+endfunc
+
 func Test_diff_rnu()
   CheckScreendump