patch 8.2.1411: when splitting a window localdir is copied but prevdir is not

Problem:    when splitting a window localdir is copied but prevdir is not.
Solution:   Also copy prevdir. (closes #6667)
diff --git a/src/window.c b/src/window.c
index 7d8122e..d2e5c5c 100644
--- a/src/window.c
+++ b/src/window.c
@@ -1385,6 +1385,8 @@
 #endif
     newp->w_localdir = (oldp->w_localdir == NULL)
 				    ? NULL : vim_strsave(oldp->w_localdir);
+    newp->w_prevdir = (oldp->w_prevdir == NULL)
+				    ? NULL : vim_strsave(oldp->w_prevdir);
 
     // copy tagstack and folds
     for (i = 0; i < oldp->w_tagstacklen; i++)