Further tree_iterator refactoring

Fix various build warnings

This fixes various build warnings on Mac and Windows (64-bit).

This adds a git_pool_freelist_item struct that makes it a little
easier to follow what's going on with the pool free list block
management code.  It is functionally neutral.

This fixes a number of issues identified by valgrind - mostly
missed free calls.  Inside valgrind, mmap() may fail which causes
some of the diff tests to fail.  This adds a fallback code path
to diff_output.c:get_workdir_content() where is the mmap() fails
the code will now try to read the file data directly into allocated
memory (which is what it would do if the data needed to be filtered
anyhow).

This updates the tree iterator internals to be more efficient.

The tree_iterator_entry objects are now kept as pointers that are
allocated from a git_pool, so that we may use git__tsort_r for
sorting (which is better than qsort, given that the tree is
likely mostly ordered already).

Those tree_iterator_entry objects now keep direct pointers to the
data they refer to instead of keeping indirect index values.  This
simplifies a lot of the data structure traversal code.

This also adds bsearch to find the start item position for range-
limited tree iterators, and is more explicit about using
git_path_cmp instead of reimplementing it.  The git_path_cmp
changed a bit to make it easier for tree_iterators to use it (but
it was barely being used previously, so not a big deal).

This adds a git_pool_free_array function that efficiently frees a
list of pool allocated pointers (which the tree_iterator keeps).
Also, added new tests for the git_pool free list functionality
that was not previously being tested (or used).

Fix workdir iterator bugs

This fixes two bugs with the workdir iterator depth check: first
that the depth was not being decremented and second that empty
directories were counting against the depth even though a frame
was not being created for them.

This also fixes a bug with the ENOTFOUND return code for workdir
iterators when you attempt to advance_into an empty directory.
Actually, that works correctly, but it was incorrectly being
propogated into regular advance() calls in some circumstances.

Added new tests for the above that create a huge hierarchy on
the fly and try using the workdir iterator to traverse it.

Advertise and support side-band-64k when calling receive-pack

Refactor iterators

Clean up some sorting function stuff including fixing qsort_r
on MinGW, common function pointer type for comparison, and basic
insertion sort implementation (which we, regrettably, fall back
on for MinGW).

Implemented push on the local transport

Given a group of case-insensitively equivalent tree iterator
entries, this ensures that the case-sensitively first trees will
be used as the representative items.  I.e. if you have conflicting
entries "A/B/x", "a/b/x", and "A/b/x", this change ensures that
the earliest entry "A/B/x" will be returned.  The actual choice
is not that important, but it is nice to have it stable and to
have it been either the first or last item, as opposed to a
random item from within the equivalent span.

handle small files in similarity metrics

Tree iterator advance was moving forward without taking the
filemode of the entries into account, equating "a" and "a/".
This makes the tree entry comparison code more easily reusable
and fixes the problem.

This fixes an off by one error for generating full paths for
tree entries in tree iterators when INCLUDE_TREES is set.  Also,
contains a bunch of small code cleanups with a couple of small
utility functions and macro changes to eliminate redundant code.

If there are case-ambiguities in the path of a case insensitive
tree iterator, it will now rewrite the entire path when it gives
the path name to an entry, so a tree with "A/b/C/d.txt" and
"a/B/c/E.txt" will give the true full paths (instead of case-
folding them both to "A/B/C/d.txt" or "a/b/c/E.txt" or something
like that.

This adds a test case for ci tree iteration when there is a name
conflict.  This points out a behavior quirk in the current version
that I'd like to fix - namely, all tree entries get mapped to one
version of the case pattern in the ci code - i.e. even if you have
A/1.txt and a/2.txt, both will be reported as a/1.txt and a/2.txt
because we only copy the name of a file at a given frame once. It
would be nice to fix this, but I'm worried about how complex that
is if you get a/B/c/1.txt and A/b/C/2.txt.  It may require a walk
up the frames whenever you advance to the next item in a blended
equivalence class.

Can't perform diff with no context lines

Previously, 0 meant default. This is problematic, as asking for 0
context lines is a valid thing to do.

Change GIT_DIFF_OPTIONS_INIT to default to three and stop treating 0
as a magic value. In case no options are provided, make sure the
options in the diff object default to 3.

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.

MinGW >= 3.14 does this automatically, but mingw-64 wants us to define
it.

Add SONAME build option to facilitate building for Android.

Optional tracing back to consumers

refs: explicitly catch leading slashes

It's somewhat common to try to write "/refs/tags/something". There is
no easy way to catch it during the main body of the function, as there
is no way to distinguish whether it's a leading slash or a double
slash somewhere in the middle.

Catch this at the beginning so we don't trigger the assert in
is_all_caps_and_underscore().

 Added build option SONAME to control whether VERSION and SOVERSION were set for the git2 target, as some platforms require that this NOT be set.

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.