patch 8.0.1394: cannot intercept a yank command

Problem:    Cannot intercept a yank command.
Solution:   Add the TextYankPost autocommand event. (Philippe Vaucher et al.,
            closes #2333)

src/fileio.c

@@ -6478,6 +6478,7 @@ buf_modname(
 /*
  * Like fgets(), but if the file line is too long, it is truncated and the
  * rest of the line is thrown away.  Returns TRUE for end-of-file.
+ * If the line is truncated then buf[size - 2] will not be NUL.
  */
     int
 vim_fgets(char_u *buf, int size, FILE *fp)
@@ -7856,6 +7857,7 @@ static struct event_name
     {"WinEnter",	EVENT_WINENTER},
     {"WinLeave",	EVENT_WINLEAVE},
     {"VimResized",	EVENT_VIMRESIZED},
+    {"TextYankPost",	EVENT_TEXTYANKPOST},
     {NULL,		(event_T)0}
 };
 
@@ -9399,6 +9401,15 @@ has_funcundefined(void)
     return (first_autopat[(int)EVENT_FUNCUNDEFINED] != NULL);
 }
 
+/*
+ * Return TRUE when there is a TextYankPost autocommand defined.
+ */
+    int
+has_textyankpost(void)
+{
+    return (first_autopat[(int)EVENT_TEXTYANKPOST] != NULL);
+}
+
 /*
  * Execute autocommands for "event" and file name "fname".
  * Return TRUE if some commands were executed.