Gives a better DWIM behaviour for --pretty=format:%gd, "stash list", and
"log -g", depending on how the starting point ("master" vs "master@{0}" vs
"master@{now}") and date formatting options (e.g. "--date=iso") are given
on the command line.

By Jeff King (4) and Junio C Hamano (1)
* jk/maint-reflog-walk-count-vs-time:
  reflog-walk: tell explicit --date=default from not having --date at all
  reflog-walk: always make HEAD@{0} show indexed selectors
  reflog-walk: clean up "flag" field of commit_reflog struct
  log: respect date_mode_explicit with --format:%gd
  t1411: add more selector index/date tests