hg-core: return Err if `offset != bytes.len()`
(D8958#inline-14994 followup 2/2)
[X] make `Index` owner of its bytes
[X] make `Index::new` return an error if `offset != bytes.len()`
Differential Revision: https://phab.mercurial-scm.org/D9107
#require unix-permissions no-root
$ hg init a
$ cd a
$ echo foo > b
$ hg add b
$ hg ci -m "b"
$ chmod -w .hg/store
$ cd ..
$ hg clone a b
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
new changesets 97310831fa1a
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ chmod +w a/.hg/store # let test clean up
$ cd b
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
checked 1 changesets with 1 changes to 1 files
$ cd ..