I just had this issue today with Jenkins 2.x.
It occurs at the very first time you start a new jenkins install and install plugins.
Probably the pipeline is installed/loaded before the git plugin in the installation process and thus you don't have access to it
java.lang.NoSuchMethodError: No such DSL method 'git' found among [archive, bat, build, catchError, checkout, deleteDir, dir, echo, emailext, error, fileExists, input, isUnix, load, mail, node, parallel, properties, pwd, readFile, retry, sh, sleep, stage, stash, step, svn, timeout, timestamps, tool, unarchive, unstash, waitUntil, withCredentials, withEnv, wrap, writeFile, ws]
Maybe after the installation wizard we should automatically trigger a restart to avoid this problem