the subversion plugin supports multiple roots and sets variables like SVN_REVISION_1. the git-plugin does the same and sets GIT_COMMIT. the suversiom-plugin sets SVN_URL_1, the git plugin sets GIT_URL, or GIT_URL_1 for the first one, depending how many there are.  there is a request JENKINS-34271 to add GIT_COMMIT_n to the git plugin, which was closed with "use pipeline plugin", without referring to more details.

it would be great to make this consistent. and document it so it can be found easily.