When showing copy information because we are duplicating contents,
for example, when performing a `diff --find-copies-harder -M100 -B100`,
then show copy from/to lines in a patch, and do not show context.
Ensure that we can also parse such patches.

Test that we can create a diff file, then parse the results and
that the two are identical in-memory.

Parse diff files into a `git_diff` structure.

Test that submodules are found when the are included in a pathspec
but have a trailing slash.

Iterator tests were split over repo::iterator and diff::iterator,
with duplication between the two.  Move them to iterator::index,
iterator::tree, and iterator::workdir.

Refactored the tree iterator to never recurse; simply process the
next entry in order in `advance`.  Additionally, reduce the number of
allocations and sorting as much as possible to provide a ~30% speedup
on case-sensitive iteration.  (The gains for case-insensitive iteration
are less majestic.)