filteredhash: split the computation of revision sets
The branch2's filteredhash combines the filtered revisions and the obsolete
ones, this will creates issue for implicit reference to heads we want to
introduce for the v3 of the branch cache format. So we isolate this logic for
alternative use.
# 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