There is a serious bug in the previous tree iterator implementation.
If case insensitivity resulted in member elements being equivalent
to one another, and those member elements were trees, then the
children of the colliding elements would be processed in sequence
instead of in a single flattened list.  This meant that the tree
iterator was not truly acting like a case-insensitive list.

This completely reworks the tree iterator to manage lists with
case insensitive equivalence classes and advance through the items
in a unified manner in a single sorted frame.

It is possible that at a future date we might want to update this
to separate the case insensitive and case sensitive tree iterators
so that the case sensitive one could be a minimal amount of code
and the insensitive one would always know what it needed to do
without checking flags.

But there would be so much shared code between the two, that I'm
not sure it that's a win.  For now, this gets what we need.

More tests are needed, though.

This standardizes iterator behavior across all three iterators
(index, tree, and working directory).  Previously the working
directory iterator behaved differently from the other two.

Each iterator can now operate in one of three modes:

1. *No tree results, auto expand trees* means that only non-
   tree items will be returned and when a tree/directory is
   encountered, we will automatically descend into it.
2. *Tree results, auto expand trees* means that results will
   be given for every item found, including trees, but you
   only need to call normal git_iterator_advance to yield
   every item (i.e. trees returned with pre-order iteration).
3. *Tree results, no auto expand* means that calling the
   normal git_iterator_advance when looking at a tree will
   not descend into the tree, but will skip over it to the
   next entry in the parent.

Previously, behavior 1 was the only option for index and tree
iterators, and behavior 3 was the only option for workdir.

The main public API implications of this are that the
`git_iterator_advance_into()` call is now valid for all
iterators, not just working directory iterators, and all the
existing uses of working directory iterators explicitly use
the GIT_ITERATOR_DONT_AUTOEXPAND (for now).

Interestingly, the majority of the implementation was in the
index iterator, since there are no tree entries there and now
have to fake them.  The tree and working directory iterators
only required small modifications.

Since the case sensitivity is moved into the respective iterators,
this removes the spoolandsort iterator code.

The iterator APIs are not currently consistent with the parameter
ordering of the rest of the codebase.  This rearranges the order
of parameters, simplifies the naming of a number of functions, and
makes somewhat better use of macros internally to clean up the
iterator code.

This also expands the test coverage of iterator functionality,
making sure that case sensitive range-limited iteration works
correctly.

[RFC] basic note iterator implementation

* fixed style issues
* use new iterator functions for git_note_foreach()

* git_note_iterator_new() - create a new note iterator
* git_note_next() - retrieves the next item of the iterator

remote push test fix

More tests (and fixes) for initializing repo from template

Disable ignore_case when writing the index to a tree

clear REUC on checkout

`git_diff_get_patch()` would unconditionally load the patch object and
then simply leak it if the user hadn't requested it. Short-circuit
loading the object if the user doesn't want it.

The rest of the plugs are simply calling the free functions of objects
allocated during the tests.

indexer: use a hashtable for keeping track of offsets

These offsets are needed for REF_DELTA objects, which encode which
object they use as a base, but not where it lies in the packfile, so
we need a list.

These objects are mostly from older packfiles, before OFS_DELTA was
widely spread. The time spent in indexing these packfiles is greatly
reduced, though remains above what git is able to do.

indexer: kill git_indexer

This was the first implementation and its goal was simply to have
something that worked. It is slow and now it's just taking up
space. Remove it and switch the one known usage to use the streaming
indexer.

Update contributing and conventions

When the examples/diff.c was written, there was not yet a revparse
API.  Now we can use it to make command line parsing way better
with less code.  Yay!

The discussion about converting some of our foreach-style APIs to
use iterator objects got me wanting to make a list of good starter
projects.  I put it in CONTRIBUTING.md and then went crazy with
updates to that file and to CONVENTIONS.md.

Allow empty config object and use it for tests

This removes assertions that prevent us from having an empty
git_config object and then updates some tests that were
dependent on global config state to use an empty config before
running anything.

Control for core.autocrlf during testing

Clone should not clean up directories it did not create

Why cdecl why?

This fixes some snprintf and vsnprintf related deprecation
warnings we've been having on Windows with recent compilers.

This removes the one-off GIT_CDECL and adds a new standard way of
doing this named GIT_STDLIB_CALL with a src/win32 specific def
when on the Windows platform.

Allows compilation in newer versions of MinGW that already defined it.