Use git client plugin 1.21.0
Use git client plugin 1.21.0

Prep for multi-branch indexing support of command line git

v1v/git-plugin,jacob-keller/git-plugin,jenkinsci/git-plugin,recena/git-plugin,kzantow/git-plugin,jacob-keller/git-plugin,martinda/git-plugin,jenkinsci/git-plugin,martinda/git-plugin,jacob-keller/git-plugin,kzantow/git-plugin,MarkEWaite/git-plugin,MarkEWaite/git-plugin,jenkinsci/git-plugin,recena/git-plugin,MarkEWaite/git-plugin,MarkEWaite/git-plugin,v1v/git-plugin,v1v/git-plugin,kzantow/git-plugin,jenkinsci/git-plugin,martinda/git-plugin,recena/git-plugin