mergestate: move binary format documentation into _readrecordsv2
This is too low-level to be the top-level documentation for mergestate. We're
restricting the top-level documentation to only be about what consumers of the
mergestate and anyone extending it need to care about.
basic test for hg debugrebuildstate
$ hg init repo
$ cd repo
$ touch foo bar
$ hg ci -Am 'add foo bar'
adding bar
adding foo
$ touch baz
$ hg add baz
$ hg rm bar
$ hg debugrebuildstate
state dump after
$ hg debugstate --nodates | sort
n 644 -1 set bar
n 644 -1 set foo
status
$ hg st -A
! bar
? baz
C foo
$ cd ..