Merge branch 'hn/reftable' into master

Preliminary clean-up of the refs API in preparation for adding a new refs backend "reftable". * hn/reftable: reflog: cleanse messages in the refs.c layer bisect: treat BISECT_HEAD as a pseudo ref t3432: use git-reflog to inspect the reflog for HEAD lib-t6000.sh: write tag using git-update-ref