Buildout remove parts when the configuration of the section has changed, by doing
its own uninstall (which remove parts), then it does a new install (instead of
doing an update, for example).
So, specifying a new revision makes buildout knowing a new section configuration,
and then removing the parts when we just wanted to update the code.
(This problem doesn't appear when a branch name is specified because section
configuration is not modified.)
So, this patch permits to the gitclone recipe to find the revision parameter
in an other section nammed [<name>-ovverride] where <name> is the name of the section calling the gitclone.
This, in order to do not remove parts each time revision has changed,
it acelerates considerably the execution time and bandwidth consumption
of the git operation.