magit-commit-at-point: never raise error

Remove the NOERROR argument and just *never* raise an error if
there isn't a commit at point.  That's what most callers need.
magit-guess-branch: remove unnecessary kludge

`magit-section-case' now always returns nil if no section matches,
so we no longer have to deal with a bogus `t' return value.
magit-guess-branch: remove unnecessary complication
magit-guess-branch: prettify
remove function magit-marked-commit

It simply returned the value of variable `magit-marked-commit' and
raised an error if that was nil.  Make the only caller deal with the
latter itself.
magit-diff-with-mark: ask for commit if there is none at point
add "Commit Mark" section

Add the "Commit Mark" section.  Its body was previously part of "Commit
Mode", but the buffer showing a commit as little to do with the commit
marked in other buffers.
cosmetics
magit-interactive-rebase: do the reading inside interactive
remove magit-set-marked-commit

Remove `magit-set-marked-commit', instead do the
work inside its only caller `magit-mark-item'.
move definition of magit-for-all-buffers
rename magit-{for-all-buffers => map-magit-buffers}
simplify magit-set-mode-line-process
rename magit-{for-all-sections => map-sections}
simplify magit-map-sections
magit-wash-sequence: correct doc-string
magit-refresh-marked-commits-in-buffer: cosmetics
Merge branch 'cleanup'