magit-log-display-author-date: adjust margin in correct window

This partially reverts f30e76945f82602622a5023c1ffca4dd6ef6d0ee.
Before that commit we were not "unnecessarily retrictive" in making
sure *current buffer* was the correct one.  Instead made sure that
the correct *window* was selected.

The change to `magit-log-create-author-date-overlay' was correct.
magit-log-{initialize,create}-author-date-overlay: move mode check to caller

It's better to do that there so that the reader of the caller knows
right away that these functions don't do anything (now are not called)
unless the mode derives from `magit-log-mode'.
add magit-log-create-author-date-overlays

Merge functions `magit-log-set-author-date-overlays' and
`magit-log-create-author-date-overlay' (singular (wrong))
into new `magit-log-create-author-date-overlays'.

Old `set' was only used by old `create'.
Also what does "set" supposed to mean?
rename magit-log-{display => refresh}-author-date

That's the term we commonly use for this action.
rename magit-log-initialize-author-date-overlay to magit-log-setup-author-date

Rename `magit-log-initialize-author-date-overlay' to
`magit-log-setup-author-date', removing the "-overlay" suffix (this
function doesn't touch any overlay) and replacing "initialize" with
"setup" (which is also not optimal but since we use the same term in
`magit-mode-setup' it makes at least some sense).
magit-log-author-{*} variables: remove useless doc-strings

Better no doc-string than one that provides no useful information.
move misplaced custom options

Previously the options `magit-merge-warn-dirty-worktree' and
`magit-log-author-date-max-length' were defined somewhere near their
use.  That's wrong because I have a section which is supposed all
options.  Move them there.  True many options are misplaced *inside*
that section, but that is a different issue.
magit-log-author-date-max-length: improve doc-string
magit-log-create-author-date-overlays: cosmetics