Files returned from version control (i.e via --outgoing or --workdir), are currently joined to cwd. This will cause failures if |mach lint| is run from anywhere other than topsrcdir. However we *do* want to join manually specified paths to cwd so things like: cd devtools && mach lint client continue to work. This patch makes sure we join the proper kind of path to the proper place. MozReview-Commit-ID: EQmRhAr3Oog