`magit-refresh-commit-buffer': pass `magit-diff-options' to git-log(1)

Take custom diff options into account when using `magit-show-commit'.

See commit a233bd4d.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #775 from praet/show-commit-diff-opts

`magit-refresh-commit-buffer': pass `magit-diff-options' to git-log(1)