revlog: rename `indexfile` to `_indexfile`
We want to make the actual location of the indexfile and location more of an
implementation details than what is is currently. In that process, we make the
attribute private.
Differential Revision: https://phab.mercurial-scm.org/D10574
# 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