rebaseline_server: constrain setADir/setBDir to consistently refer to single directories, not lists Before this CL, we supported multiple setADirs/setBDirs for some operations but not others... now we consistently support a single pair of directories. NOTRY=true R=stephana@google.com TBR=stephana Author: epoger@google.com Review URL: https://codereview.chromium.org/480293002