combo-layer: combine trees via replacement objects
Instead of local graft entries rooting the imported branches in the
shared root commit, use replacement objects.
The advantage is that they get moved around by "git push" and "git
fetch", so everyone has the same, nicer view with everything starting
at the beginning of the combined repository.
If undesired, these objects can be removed with "git replace".
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
