magit-wash-raw-diff: never delay parsing hunks

Close #1128.  See that issue for an explanation.

For now just comment the offending code, it will most likely be removed
later on.  The other code involved in this and sprinkled across Magit is
left as is, as without the part commented here it cannot do any harm.
magit-show: erase-buffer before recreating content
magit-show: make buffer read-only
sort custom options into multiple custom groups and rearrange defintions

The `magit' custom group has gotten to full, split into several
subgroups.  Also rearrange the definitions of custom options in
`magit.el' into sections that more or less match the groups.

In other libraries add a new "Options" section unless that already
existed.  Move misplaced option definitions into that section.  Also
split the remainder of these libraries into sections where necessary,
to avoid adding a "(rest)" section (which indicates that it still has
to be split into multiple sections).
magit-blame-map: don't define as a constant
magit-blame-map: don't set menu string
magit-blame-sha1: cosmetics
add gitattributes-mode to magit-modes custom group
rename option magit-{ => process}-quote-curly-braces
magit-status: call magit-save-some-buffers

It used to do that but this got lost somewhere along the way.
magit-save-some-buffers: adjust doc-string

Update the option's doc-string; `magit-status' isn't the
only command that calls function `magit-save-some-buffers'.
rename function magit-{git => process}-quote-arguments
magit-process-quote-arguments: move definition
magit-{call, start}-git: call magit-process-quote-arguments

The processors of these functions used to do that
but this got lost somewhere along the way.
Merge branch 'pr/options'
magit-show-xref-buttons: fix typo in doc-string
magit-checkout-branch-at-point: remove unnecessary function

Instead add a `branch' case to `magit-visit-item'.
magit-insert-rebase-sequence: new function

I intend to further improve this.
magit-insert-unpulled-or-recent-commits: new function

Do not add it to the default value of `magit-status-sections-hook',
as I intend to make bigger changes to this and other log sections.
Revert "magit-wash-raw-diff: never delay parsing hunks"

This reverts commit d976c0dc5b30dd761d2df657c8c9095656c036d7.