patch 8.1.0677: look-behind match may use the wrong line number

Problem:    Look-behind match may use the wrong line number. (Dominique Pelle)
Solution:   Use the line number in regsave instead of the one in behind_pos,
            we may be looking at the previous line. (closes #3749)

src/regexp.c

@@ -5582,7 +5582,7 @@ regmatch(
 			if (has_mbyte)
 			{
 			    char_u *line =
-					 reg_getline(behind_pos.rs_u.pos.lnum);
+				  reg_getline(rp->rs_un.regsave.rs_u.pos.lnum);
 
 			    rp->rs_un.regsave.rs_u.pos.col -=
 				(*mb_head_off)(line, line

src/version.c

@@ -799,6 +799,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    677,
 /**/
     676,
 /**/