finish rename magit-{ => diff-}show-or-scroll-{up,down}

Fixes #1506.
magit-reset-index: Fix typo
magit-reset*: Fix ambiguous argument

When both a revision and a file have the same name,
`magit-reset{,head,soft,hard}' would fail with the following error.

  fatal: ambiguous argument 'NAME': both revision and filename
  Use '--' to separate paths from revisions, like this:
  'git <command> [<revision>...] -- [<file>...]'

Add '--' to all commands to make it clear that the argument is a
revision. (This is already used in `magit-reset-index' and is the same
fix used in commit c5a6cc2577 before the reset functions were
rewritten).
Merge pull request #1508 from kyleam/n/typo

magit-reset-index: Fix typo
Merge pull request #1509 from kyleam/n/fix-ambiguous-reset

magit-reset*: Fix ambiguous argument
magit-rebase{,-interactive}: remove an old trick

Back in the days `magit-rebase' and `magit-rebase-interactive' were
invoked directly, not through the rebase popup.  So we had to make
sure the user did not try to begin rebasing when another sequence
was already in progress.  We don't have to do that anymore.
magit-rebase{,-onto,-interactive}: improve doc-strings
magit-bisect-{good,skip,bad}: move definitions to magit-bisect.el