Merge branch 'jz/rev-list-exclude-first-parent-only'

"git log" and friends learned an option --exclude-first-parent-only
to propagate UNINTERESTING bit down only along the first-parent
chain, just like --first-parent option shows commits that lack the
UNINTERESTING bit only along the first-parent chain.

* jz/rev-list-exclude-first-parent-only:
  git-rev-list: add --exclude-first-parent-only flag

diff --cc revision.h
Simple merge

diff --cc t/t6012-rev-list-simplify.sh
Simple merge