patch 8.2.3363: when :edit reuses the current buffer the alternate file is set

Problem:    When :edit reuses the current buffer the alternate file is set to
            the same buffer.
Solution:   Only set the alternate file when not reusing the buffer.
            (closes #8783)
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index d30db91..021296d 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -2648,6 +2648,8 @@
      */
     if (other_file)
     {
+	int prev_alt_fnum = curwin->w_alt_fnum;
+
 	if (!(flags & (ECMD_ADDBUF | ECMD_ALTBUF)))
 	{
 	    if ((cmdmod.cmod_flags & CMOD_KEEPALT) == 0)
@@ -2691,6 +2693,10 @@
 	}
 	if (buf == NULL)
 	    goto theend;
+	if (curwin->w_alt_fnum == buf->b_fnum && prev_alt_fnum != 0)
+	    // reusing the buffer, keep the old alternate file
+	    curwin->w_alt_fnum = prev_alt_fnum;
+
 	if (buf->b_ml.ml_mfp == NULL)		// no memfile yet
 	{
 	    oldbuf = FALSE;