merge-recursive: fix longstanding bug in merging symlinks

Commit 3af244ca added unlink(2) before running symlink(2) to
update the working tree with the merge result, but it was
unlinking a wrong path.  This resulted in loss of the path
pointed by a symlink.

Signed-off-by: Junio C Hamano <junkio@cox.net>

merge-recursive.c

@@ -589,7 +589,7 @@ static void update_file_flags(const unsigned char *sha,
 			memcpy(lnk, buf, size);
 			lnk[size] = '\0';
 			mkdir_p(path, 0777);
-			unlink(lnk);
+			unlink(path);
 			symlink(lnk, path);
 		} else
 			die("do not know what to do with %06o %s '%s'",