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.


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Base idea is that I wanted to see merge diff in tig's view-diff.
It is enabled by passing `-m --first-parent` to git show options.
But tig's diff-options was passed to view-diff and view-main.
So, passing show options only into view-diff is impossible.