hg-core: make `Index` owner of its bytes (D8958#inline-14994 followup 1/2)
Prevent building `Index` every time it is needed. It was a bad idea anyway.
When `Index::new` will return `Result` it will avoid things like `Revlog::len`
returning `Result<usize>` instead of `usize`.
[X] make `Index` owner of its bytes
[ ] make `Index::new` return an error if `offset != bytes.len()`
Differential Revision: https://phab.mercurial-scm.org/D9106
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif