`magit-commit': fully expand "Staged changes" section

Commit 64225dbc added support for auto-expanding the "Staged changes"
section when committing, but did so using `magit-expand-section' which
only expands the current section, not its subsections, so when
individual chunks were collapsed, they remained in that state.

Use `magit-show-level' instead.

Signed-off-by: Pieter Praet <pieter@praet.org>