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
4 files changed