dirstate: add some debug output when writing the dirstate stable

dirstate: add some debug output when writing the dirstate

This helps debugging Mercurial behavior in the dirstate-v2 case.

--- a/rust/hg-core/src/repo.rs	Tue Jan 31 13:16:39 2023 +0100
+++ b/rust/hg-core/src/repo.rs	Tue Feb 28 00:04:32 2023 +0100
@@ -481,7 +481,10 @@
             //   https://github.com/openzfs/zfs/issues/13370
             //
             if !append {
+                log::trace!("creating a new dirstate data file");
                 options.create_new(true);
+            } else {
+                log::trace!("appending to the dirstate data file");
             }
 
             let data_size = (|| {