Merge branch 'ps/transactional-update-ref-stdin'

"git update-ref --stdin" learned a handful of new verbs to let the user control ref update transactions more explicitly, which helps as an ingredient to implement two-phase commit-style atomic ref-updates across multiple repositories. * ps/transactional-update-ref-stdin: update-ref: implement interactive transaction handling update-ref: read commands in a line-wise fashion update-ref: move transaction handling into `update_refs_stdin()` update-ref: pass end pointer instead of strbuf update-ref: drop unused argument for `parse_refname` update-ref: organize commands in an array strbuf: provide function to append whole lines git-update-ref.txt: add missing word refs: fix segfault when aborting empty transaction