BT: don't export 'revision' anymore

Because on DVCS with branches, history is not always linear, we must stop
using increasing integer for revisions because they cause too many conflicts
and a higher revision number does not necessarily means the a BT newer.
They're now a cryptographic hash of all the contents of the Business Template,
and they're computed automatically:
- by genbt5list
- at download
- at building (by default)
- at export
Use default __hash__ method for ERP5 objects

If an object has no uid yet and has no acquisition context to the portal,
getUid() fails.

Default is derived from id(aq_base(ob)) which is even more efficient.

The method on XMLObject was also redundant because it inherited the same one
from Base.

(cherry picked from commit 347804af6ed8e114c6195cca128afea946240043)
BT: when downloading from VCS, add a history comment with revision information
Remove all bt/revision files
Merge branch to remove 'bt/revision' files