All the objects which will will be eventually transversable from
a revision pool (commits, trees, etc) now inherit from the
'git_revpool_object' structure which identifies them with their
own OID.

Furthermore, the 'git_revpool_table' and related functions have
been added, which allow for constant time lookup (hash table)
of the loaded revpool objects based on their OID.

Signed-off-by: Vicent Marti <tanoku@gmail.com>
Signed-off-by: Andreas Ericsson <ae@op5.se>