git-arc: allow word splitting in build_commit_list()

git-rev-list expects commits to be listed separately, allow word
splitting and disable shellcheck SC2086 when using git-rev-list
to build the commit list.

Fixes: 4fd0c6ab1a9e ("Fix most shellcheck warnings in git-arc.sh")
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D31838