Don't use branch name in Git buildnumber.

As we want to build the artifacts from a clean checkout of a tag, we are
in 'detached head' state, without a branch name.

The build number will retain the most important thing for traceability:
the last commit SHA id.
Improved release instructions.

The site for reviewing should not be put in the staging area (otherwise
it may be published too early as this area is shared among all commons
components).

The VOTE mail message should contain the commit ID the tag points at,
for better traceability.