The "git range-diff" command learned "--(left|right)-only" option
to show only one side of the compared range.

* js/range-diff-one-side-only:
  range-diff: offer --left-only/--right-only options
  range-diff: move the diffopt initialization down one layer
  range-diff: combine all options in a single data structure
  range-diff: simplify code spawning `git log`
  range-diff: libify the read_patches() function again
  range-diff: avoid leaking memory in two error code paths