bundle2: make the "hgtagsfnodes" part advisory
This bundle2 part is about helping the client to warms its cache. There is no
reason for it to be mandatory.
So we mark it advisory.
Set up
$ hg init repo
$ cd repo
$ echo a > a
$ hg add a
$ hg commit -m a
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[10]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate