magit-{log-reflog-re,refresh-reflog-buffer}: better format

Drop the unnecessary "*" prefix.  Do so not only in the format and
regexp, but also in the display.  And replace the unnecessarily ugly
separators in the format and regexp.
magit-log-reflog-re: deal with git-merge inconsistency

Other commands put the revs and refs they deal with after the colon.
Before the colon goes the command and in some cases details about the
command, i.e. what variant of the command is used or in what mode
command was performed.

  rebase -i (finish): commit message

`git-merge' confuses the order and the position of the colon.

  merge branch-name: Fast-forward

Adjust the regexp to pretend the colon comes before the branch-name.
Ignore the detail part for now.
magit-reflog-subject-re: remove unnecessary complication

"-" doesn't have a special meaning at the beginning of '[...]'.
And right after '^' is still "at the beginning".
magit-reflog-subject-re: remove strange submatch

I am not aware of any git command that would produce this "status" part
and this might actually match to much when `update-rev' is used directly.
magit-refresh-reflog-buffer: use git-reflog

Use the git command `reflog' to display the reflog,
not `log --walk-reflogs'.  That's ... simpler.
reflog: also insert reflog identifier

but abbreviate it to just a number.
magit-log-margin-set-timeunit-width: new function

Extract `magit-log-margin-set-timeunit-width' from
`magit-refresh-log-buffer' so that it can be used elsewhere to.

Actually we should refactor the involved variables so that this only
has to be done when setting the "main" variable.  But there are other
things that should be improved here.  For users sake delay addressing
this particular point to avoid changing the options several times.
reflog: support displaying the log margin
Merge branch 'pr/reflog'

Conflicts:
	magit.el