Enlarge the effective space on MR diffs

Even after !42821 (merged) (#254197 (closed)) merged, the effective space for MR diffs on MR is still small on mobile/tablet screens. One of the main reason is too wider line number columns, which can be made more compact.

as-is (13.9 or 13.10-pre)

cf. the screenshot as of 13.4)

to-be

TBU

part of &331