hgview #20866 offer to use showfunction for diff [validation pending]

Ideally hgview would honor the [diff] section of hgrc, which can be used to pass options to hg diff.

One option I find lacking in the current diff is showfunction which is useful to provide context when browsing only from diff, but conceivably the -U nb_context_lines and the various whitespace handling options would be useful.

done in1.8.0
load left0.000
closed by#1e90c4932c68 honor the [diff] section of hgrc (closes #20866)