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.)

Disambiguate the reset and reset_range functions.  Now reset_range
with a NULL path will clear the start or end; reset will leave the
existing start and end unchanged.

Instead of copying over the data into the individual entries, point to
the originals, which are already in a format we can use.

We cannot know from looking at .gitmodules whether a directory is a
submodule or not. We need the index or tree we are comparing against to
tell us. Otherwise we have to assume the entry in .gitmodules is stale
or otherwise invalid.

Thus we pass the index of the repository into the workdir iterator, even
if we do not want to compare against it. This follows what git does,
which even for `git diff <tree>`, it will consider staged submodules as
such.

The diff code was using an "ignored_prefix" directory to track if
a parent directory was ignored that contained untracked files
alongside tracked files. Unfortunately, when negative ignore rules
were used for directories inside ignored parents, the wrong rules
were applied to untracked files inside the negatively ignored
child directories.

This commit moves the logic for ignore containment into the workdir
iterator (which is a better place for it), so the ignored-ness of
a directory is contained in the frame stack during traversal.  This
allows a child directory to override with a negative ignore and yet
still restore the ignored state of the parent when we traverse out
of the child.

Along with this, there are some problems with "directory only"
ignore rules on container directories.  Given "a/*" and "!a/b/c/"
(where the second rule is a directory rule but the first rule is
just a generic prefix rule), then the directory only constraint
was having "a/b/c/d/file" match the first rule and not the second.
This was fixed by having ignore directory-only rules test a rule
against the prefix of a file with LEADINGDIR enabled.

Lastly, spot checks for ignores using `git_ignore_path_is_ignored`
were tested from the top directory down to the bottom to deal with
the containment problem, but this is wrong. We have to test bottom
to top so that negative subdirectory rules will be checked before
parent ignore rules.

This does change the behavior of some existing tests, but it seems
only to bring us more in line with core Git, so I think those
changes are acceptable.

This makes the lock management on the index a little bit broader,
having a number of routines hold the lock across looking up the
item to be modified and actually making the modification.  Still
not true thread safety, but more pure index modifications are now
safe which allows the simple cases (such as starting up a diff
while index modifications are underway) safe enough to get the
snapshot without hitting allocation problems.

As part of this, I simplified the allocation of index entries to
use a flex array and just put the path at the end of the index
entry.  This makes every entry self-contained and makes it a
little easier to feel sure that pointers to strings aren't
being accidentally copied and freed while other references are
still being held.