reflog-walk: clean up "flag" field of commit_reflog struct

reflog-walk: clean up "flag" field of commit_reflog struct

When we prepare to walk a reflog, we parse the specification
and pull some information from it, such as which reflog to
look in (e.g., HEAD), and where to start (e.g., HEAD@{10} or
HEAD@{yesterday}). The resulting struct has a "recno" field
to show where in the reflog we are starting. It also has a
"flag" field; if true, it means the recno field came from
parsing a date like HEAD@{yesterday}.

There are two problems with this:

  1. "flag" is an absolutely terrible name, as it conveys
     nothing about the meaning

  2. you can tell "HEAD" from "HEAD@{yesterday}", but you
     can't differentiate "HEAD" from "HEAD{0}"

This patch converts the flag into a tri-state (and gives it
a better name!).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>