dirstate-tree: Downgrade `&mut Node` to `&Node` in status and serialization
Mutable access is not used, and upcoming changes will make it more costly
(with copy-on-write nodes that can be read from disk representation)
Differential Revision: https://phab.mercurial-scm.org/D10745
# Files that just need to be migrated to the formatter.
# Do not add new files here!
mercurial/cext/manifest.c
mercurial/cext/osutil.c
# Vendored code that we should never format:
syntax: glob
contrib/python-zstandard/**.c
contrib/python-zstandard/**.h
hgext/fsmonitor/pywatchman/**.c
mercurial/thirdparty/**.c
mercurial/thirdparty/**.h
mercurial/pythoncapi_compat.h