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
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed