Merge branch 'jc/reflog-doc'

Document rules to use GIT_REFLOG_ACTION variable in the scripted Porcelain. git-rebase--interactive locally violates them, but it is a leaf user that does not call out to or dot-source other scripts, so it does not urgently need to be fixed. * jc/reflog-doc: setup_reflog_action: document the rules for using GIT_REFLOG_ACTION