patch 9.0.1583: get E304 when using 'cryptmethod' "xchacha20v2"

Problem:    Get E304 when using 'cryptmethod' "xchacha20v2". (Steve Mynott)
Solution:   Add 4th crypt method to block zero ID check.  Avoid syncing a swap
            file before reading the file. (closes #12433)
diff --git a/src/buffer.c b/src/buffer.c
index a47342a..330b3b9 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -218,6 +218,10 @@
 	return FAIL;
     }
 
+    // Do not sync this buffer yet, may first want to read the file.
+    if (curbuf->b_ml.ml_mfp != NULL)
+	curbuf->b_ml.ml_mfp->mf_dirty = MF_DIRTY_YES_NOSYNC;
+
     // The autocommands in readfile() may change the buffer, but only AFTER
     // reading the file.
     set_bufref(&old_curbuf, curbuf);
@@ -298,6 +302,11 @@
 	    retval = read_buffer(TRUE, eap, flags);
     }
 
+    // Can now sync this buffer in ml_sync_all().
+    if (curbuf->b_ml.ml_mfp != NULL
+	    && curbuf->b_ml.ml_mfp->mf_dirty == MF_DIRTY_YES_NOSYNC)
+	curbuf->b_ml.ml_mfp->mf_dirty = MF_DIRTY_YES;
+
     // if first time loading this buffer, init b_chartab[]
     if (curbuf->b_flags & BF_NEVERLOADED)
     {