Merge branch 'jc/prune-all'

We used the approxidate() parser for "--expire=<timestamp>" options of various commands, but it is better to treat --expire=all and --expire=now a bit more specially than using the current timestamp. Update "git gc" and "git reflog" with a new parsing function for expiry dates. * jc/prune-all: prune: introduce OPT_EXPIRY_DATE() and use it api-parse-options.txt: document "no-" for non-boolean options git-gc.txt, git-reflog.txt: document new expiry options date.c: add parse_expiry_date()