Merge branch 'dt/refs-backend-preamble'

In preparation for allowing different "backends" to store the refs in a way different from the traditional "one ref per file in $GIT_DIR or in a $GIT_DIR/packed-refs file" filesystem storage, reduce direct filesystem access to ref-like things like CHERRY_PICK_HEAD from scripts and programs. * dt/refs-backend-preamble: git-stash: use update-ref --create-reflog instead of creating files update-ref and tag: add --create-reflog arg refs: add REF_FORCE_CREATE_REFLOG flag git-reflog: add exists command refs: new public ref function: safe_create_reflog refs: break out check for reflog autocreation refs.c: add err arguments to reflog functions