vim patches 7.4.955/974/975/989. #3919

Helped by @Shougo.

vim-patch:7.4.955
vim-patch:7.4.974
vim-patch:7.4.975
vim-patch:7.4.989

Port upstream vim patches 955, 974, 975 and 989. Mark patches
964, 968, 970, and 971, and 982 as NA. Update patch list to 1022.

patch 7.4.955
Problem:    Vim doesn't recognize .pl6 and .pod6 files.
Solution:   Recognize them as perl6 and pod6. (Mike Eve)

patch 7.4.974
Problem:    When using :diffsplit the cursor jumps to the first line.
Solution:   Put the cursor on the line related to where the cursor was before
            the split.

patch 7.4.975
Problem:    Using ":sort" on a very big file sometimes causes text to be
            corrupted. (John Beckett)
Solution:   Copy the line into a buffer before calling ml_append().

patch 7.4.989
Problem:    Leaking memory when hash_add() fails. Coverity error 99126.
Solution:   When hash_add() fails free the memory.

778 marked as not NA as it will be needed once vim patch 754 is merged

Marked as NA:
964 test 87 was deleted
968 tests 86/87 were deleted
970 guarded by: `# if defined(FEAT_GUI_GTK) || defined(PROTO`
    and is inside a function that no longer exists
971 function table already sorted correctly
982 marked as NA because Neovim tests are only specified in exactly one location

1 files changed, 14 insertions, 2 deletions

diff --git a/src/nvim/diff.c b/src/nvim/diff.c
index d311588ab4..ce79158050 100644
--- a/src/nvim/diff.c
+++ b/src/nvim/diff.c
@@ -1007,6 +1007,7 @@ theend:
 void ex_diffsplit(exarg_T *eap)
 {
   win_T *old_curwin = curwin;
+  buf_T *old_curbuf = curbuf;
 
   // don't use a new tab page, each tab page has its own diffs
   cmdmod.tab = 0;
@@ -1020,8 +1021,19 @@ void ex_diffsplit(exarg_T *eap)
     // split must have worked
     if (curwin != old_curwin) {
       // Set 'diff', 'scrollbind' on and 'wrap' off.
-      diff_win_options(curwin, TRUE);
-      diff_win_options(old_curwin, TRUE);
+      diff_win_options(curwin, true);
+      if (win_valid(old_curwin)) {
+        diff_win_options(old_curwin, true);
+
+        if (buf_valid(old_curbuf)) {
+          // Move the cursor position to that of the old window.
+          curwin->w_cursor.lnum = diff_get_corresponding_line(
+              old_curbuf,
+              old_curwin->w_cursor.lnum,
+              curbuf,
+              curwin->w_cursor.lnum);
+        }
+      }
     }
   }
 }