for erp5 git repository, take into account revision in updates

stack/erp5.cfg

@@ -193,7 +193,7 @@ repository = http://git.erp5.org/repos/erp5.git
 branch = master
 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
+update-command = cd ${:location} && ${git:location}/bin/git pull --quiet && if [ -n ${:revision} ]; then cd ${:location} && ${git:location}/bin/git reset --quiet --hard ${:revision} ; fi
 
 [products]
 # XXX: ERP5 related products are not defined as python distributions, so it is