Fix revision test when cloning erp5 git repo

stack/erp5.cfg

@@ -225,8 +225,8 @@ stop-on-error = true
 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 && if [ -n ${:revision} ]; then cd ${:location} && ${git:location}/bin/git reset --quiet --hard ${:revision} ; fi
+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} && ${git:location}/bin/git pull --quiet && if [ -n "${: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