patch 8.1.2289: after :diffsplit closing the window does not disable diff
Problem: After :diffsplit closing the window does not disable diff.
Solution: Add "closeoff" to 'diffopt' and add it to the default.
diff --git a/src/proto/diff.pro b/src/proto/diff.pro
index b18c59b..d14907e 100644
--- a/src/proto/diff.pro
+++ b/src/proto/diff.pro
@@ -19,6 +19,7 @@
int diffopt_changed(void);
int diffopt_horizontal(void);
int diffopt_hiddenoff(void);
+int diffopt_closeoff(void);
int diff_find_change(win_T *wp, linenr_T lnum, int *startp, int *endp);
int diff_infold(win_T *wp, linenr_T lnum);
void nv_diffgetput(int put, long count);