magit-log-read-args: drop --graph when -G or --grep is set
add "LIBRARY.el ends soon" section headers near end of all libraries

This is indeed silly, but so is adding "LIBRARY.el ends here" (in the
age of version control), and that is a convention!  The reason I am
doing this now, is that I once more have yanked a new section before
"LIBRARY.el ends here" while all existing sections were collapsed
(committed but fixed before being pushed).  Theoretically I know doing
so results in the new section being placed after the `provide' form and
file-local variables, but it seems I always forget when I actually do
insert a section right before the library end, so let's make sure that
doesn't cause any harm.
magit-extras.el: fix typo in comment
magit-add-change-log-entry{,-other-window}: move to magit-extras.el

Users and projects that have fully embraced Git don't use ChangeLog
files, they use commit messages.  Since their are users and projects
that have not done so, the functions `magit-add-change-log-entry' and
`magit-add-change-log-entry-other-window' still do server a purpose.
But these functions are not essential to Magit and are therefor moved
to `magit-extras.el'.
magit-gitignore{,locally}: various improvements

Offer all files and patterns that make sense for completion.  And only
those.  No longer require a prefix argument to allow picking a pattern
instead of the file at point.  No longer offer to remove a tracked file
before ignoring it, instead default to offer a pattern that matches
files with the same suffix.

Replace the utility function `magit-ignore-edit-string' with more
`magit-gitignore-read-args' and remove the utility function
`magit-gitignore-file' (integrated into `magit-gitignore').

Define these functions in `magit-extras.el' instead of in `magit.el'.
magit-describe-section: move definition
magit-read-string: new function
magit-read-local-branch: add DEFAULT and EXCLUDE arguments
magit-read-remote: always require match
magit-copy-as-kill: also handle `diffstat's
magit-refresh-reflog-buffer: fix recent bug
magit-log-toggle-margin: bugfix
magit-revert-buffers: reduce nesting
magit-refresh-all: correct doc-string