magit-process-setup: bugfix

Do not attempt to elide standard arguments when the process isn't "git"
or when `magit-git-standard-options' is nil.  Fixes #1513.
magit-insert-untracked-files-1: fix indentation
magit-insert-untracked-files-1: work around Git bug

Also see #1512.
magit-insert-untracked-files-1: cd to list file in subdirectory

To list content of directory whose name contain [], we have to cd in
them first. (closes #1512).
Merge pull request #1515 from vanicat/t/untracked-dir-with-bracket

magit-insert-untracked-files-1: cd to list file in subdirectory