make the revision of repository configurable

stack/erp5.cfg

@@ -191,7 +191,8 @@ location = ${buildout:parts-directory}/${:_buildout_section_name_}
 stop-on-error = true
 repository = http://git.erp5.org/repos/erp5.git
 branch = master
-command = ${git:location}/bin/git clone --quiet -b ${:branch} ${:repository} ${:location}
+revision = 
+command = ${git:location}/bin/git clone --quiet -b ${:branch} ${:repository} ${:location} && if [ -n ${:revision} ]; then cd ${:location} && ${git:location}/bin/git reset --quiet --hard ${:revision} ; fi
 update-command = cd ${:location} && ${git:location}/bin/git pull --quiet
 
 [products]