revlog: have an explicit "pack_header" method
Having to pass the version header when retrieving the binary version of every
single entry is a bit silly. So we extract that special logic in its own method.
This also prepare the move to newer revlog format, not storing the header within
an actual entry…
Differential Revision: https://phab.mercurial-scm.org/D10510
$ hg init repo
$ cd repo
$ touch foo
$ hg ci -Am 'add foo'
adding foo
$ hg up -C null
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
this should be stored as a delta against rev 0
$ echo foo bar baz > foo
$ hg ci -Am 'add foo again'
adding foo
created new head
$ hg debugindex foo
rev linkrev nodeid p1 p2
0 0 b80de5d13875 000000000000 000000000000
1 1 0376abec49b8 000000000000 000000000000
$ cd ..