rTOMMY8d7e01d292da introduced the support for multi-branch projects by default,
while rTOMMYa95280823e1b put this feature behind a guard, requiring to set
the JENKINS_MULTI_BRANCH environment variable.
For tommy_ci, we actually want this so when staging area is enabled,
we could use branches instead of or in complement to the Phabricator plugin.
For tommy_cd, only the main branch is considered, so we don't need it.
Ref T1441.