Merge branch 'jk/maint-reflog-walk-count-vs-time' (early part) into next

Merge branch 'jk/maint-reflog-walk-count-vs-time' (early part) into next

"git log -g --format='%gd: %gs' refs/stash" (or worse yet, the same
command line with refs/stash@{0}) was affected by log.date=iso settings
and failed to show stash@{0}, stash@{1},... in the output.  Clarify the
DWIM rules to give the most precedence to how the starting point is
specified (i.e. stash@{0} vs stash@{now}), and then use the presense
of "--date=<format>" command line option as the secondary clue to decide
whether reflog entries are shown with numbers or timestamps.

By Jeff King
* 'jk/maint-reflog-walk-count-vs-time' (early part):
  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