patch 8.2.0260: several lines of code are duplicated

Problem:    Several lines of code are duplicated.
Solution:   Move duplicated code to a function. (Yegappan Lakshmanan,
            closes #5330)
diff --git a/src/regexp_bt.c b/src/regexp_bt.c
index 78be2b5..0ed3711 100644
--- a/src/regexp_bt.c
+++ b/src/regexp_bt.c
@@ -4854,17 +4854,7 @@
     proftime_T	*tm,		// timeout limit or NULL
     int		*timed_out)	// flag set on timeout or NULL
 {
-    rex.reg_match = NULL;
-    rex.reg_mmatch = rmp;
-    rex.reg_buf = buf;
-    rex.reg_win = win;
-    rex.reg_firstlnum = lnum;
-    rex.reg_maxline = rex.reg_buf->b_ml.ml_line_count - lnum;
-    rex.reg_line_lbr = FALSE;
-    rex.reg_ic = rmp->rmm_ic;
-    rex.reg_icombine = FALSE;
-    rex.reg_maxcol = rmp->rmm_maxcol;
-
+    init_regexec_multi(rmp, win, buf, lnum);
     return bt_regexec_both(NULL, col, tm, timed_out);
 }