diff creates inventory objects

in diff.py, line 508, when content differs it delegates to inventory's
diff method. This adds several seconds to trivial diffs in moderate to
large trees.

This is kindof strange as the inventory cannot access content anyway,
and for trees whose inventory is not structured like our in memory one,
expensive.

We could make the thunked inventory object lazier, but really I think
that _maybe_diff_file_or_symlink should just have a case statement -
around (kinda, kindb) - and use the tree apis directly.

-Rob

 affects /products/bzr
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