Fix diff-options to be considerated only in view-diff

diff-options was considerated not only in view-diff but also view-main.
So, it is impossible to set `git show` option. But tigrc.5.html says

    diff-options(string)
         A space separate string of diff options to use in the diff view.

So, diff-options should be considerated only in view-diff.
This commit fixes the issue.