revlog: store new index entries as binary
For a pure-Python unbundle of the current NetBSD test repository, this
results in a 10% peak RSS reduction. Using the C revlog index, it shows
25% peak RSS reduction. This is a direct result of avoiding at least
8 objects per new changeset or 200 Bytes+ on AMD64.
Differential Revision: https://phab.mercurial-scm.org/D9162
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true