patch 8.2.1967: the session file does not restore the alternate file

Problem:    The session file does not restore the alternate file.
Solution:   Add ":balt".  Works like ":badd" and also sets the buffer as the
            alternate file.  Use it in the session file. (closes #7269,
            closes #6714)
diff --git a/src/session.c b/src/session.c
index 44976e8..9731bd3 100644
--- a/src/session.c
+++ b/src/session.c
@@ -397,6 +397,19 @@
 	}
     }
 
+    if (wp->w_alt_fnum)
+    {
+	buf_T *alt = buflist_findnr(wp->w_alt_fnum);
+
+	// Set the alternate file.
+	if (alt != NULL
+		&& alt->b_fname != NULL
+		&& *alt->b_fname != NUL
+		&& (fputs("balt ", fd) < 0
+		|| ses_fname(fd, alt, flagp, TRUE) == FAIL))
+	    return FAIL;
+    }
+
     // Local mappings and abbreviations.
     if ((*flagp & (SSOP_OPTIONS | SSOP_LOCALOPTIONS))
 					 && makemap(fd, wp->w_buffer) == FAIL)