`magit--bisect-last-pos': properly set prop 'permanent-local to t

Commit 8ce80095 intended to define `magit--bisect-last-pos' as a
permanent buffer-local variable by copy-pasting and adjusting the
sexps which do so for `magit--bisect-info', but forgot to adjust
the line which is supposed to set its 'permanent-local property to t.

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-bisect-run': properly set `buffer' and `process'

In an effort to limit the usage of `magit-process-buffer-name',
commit 54f2e7c2 modified `magit-bisect-run' to set `process' and
`buffer' based on the return value of `magit-run-git-async'.

However, `magit-run-git-async' uses `magit-run-git*', which uses
`magit-run*', which returns its `successp' variable, which is either
t or nil, *not* a process object, so `magit-bisect-run' fails when
it tries to set `buffer' by running `process-buffer' on `process':

  Debugger entered--Lisp error: (wrong-type-argument processp t)
  [...]

Fixes #891.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #892 from praet/bisect

Bisect