Ended up migrating a bunch of upstream functions as well
including vector, attr_file, and odb in order to get this
to work right.

This also includes droping `git_buf_lasterror` because it makes no sense
in the new system. Note that in most of the places were it has been
dropped, the code needs cleanup. I.e. GIT_ENOMEM is going away, so
instead it should return a generic `-1` and obviously not throw
anything.

Yes, this is error handling solely for `refs.c`, but some of the
abstractions leak all ofer the code base.

Obviously all the old throw routines are still in place, so we can
gradually port over.

This reverts the changes to the GIT_STATUS constants and adds a
new enumeration to describe the type of change in a git_diff_delta.
I don't love this solution, but it should prevent strange errors
from occurring for now.  Eventually, I would like to unify the
various status constants, but it needs a larger plan and I just
wanted to eliminate this breakage quickly.

This clears up the memory leaks that valgrind seems to find on
my machine.

It turns out that commit 31e9cfc4cbcaf1b38cdd3dbe3282a8f57e5366a5
did not fix the GIT_USUSED behavior on all platforms.  This commit
walks through and really cleans things up more thoroughly, getting
rid of the unnecessary stuff.

To remove the use of some GIT_UNUSED, I ended up adding a couple
of new iterators for hashtables that allow you to iterator just
over keys or just over values.

In making this change, I found a bug in the clar tests (where we
were doing *count++ but meant to do (*count)++ to increment the
value).  I fixed that but then found the test failing because it
was not really using an empty repo.  So, I took some of the code
that I wrote for iterator testing and moved it to clar_helpers.c,
then made use of that to make it easier to open fixtures on a
per test basis even within a single test file.

This is a major reorganization of the diff code.  This changes
the diff functions to use the iterators for traversing the
content.  This allowed a lot of code to be simplified.  Also,
this moved the functions relating to outputting a diff into a
new file (diff_output.c).

This includes a number of other changes - adding utility
functions, extending iterators, etc. plus more tests for the
diff code.  This also takes the example diff.c program much
further in terms of emulating git-diff command line options.

Some changes that merged cleanly actually broke the unit
tests, so this fixes them.

This is an initial version of git_diff_workdir_to_index.  It
also includes renaming some structures and some refactoring
of the existing code so that it could be shared better with
the new function.

This is not complete since it needs a rebase to get some
new odb functions from the upstream branch.

Once I added tests for the whitespace handling options of
diff, I realized that there were some bugs.  This fixes
those and adds the new tests into the test suite.

* Implemented git_diff_index_to_tree
* Reworked git_diff_options structure to handle more options
* Made most of the options in git_diff_options actually work
* Reorganized code a bit to remove some redundancy
* Added option parsing to examples/diff.c to test most options

File mode flags are not all defined on WIN32, but since git
is so rigid in how it uses file modes, there is no reason not
to hard code a particular value.  Also, this is only used in
the git_diff_print_compact helper function, so it is really
really not important.

This fixes several bugs, updates tests and docs, eliminates the
FILE* assumption in favor of printing callbacks for the diff patch
formatter helpers, and adds a "diff" example function that can
perform a diff from the command line.

This reworks the diff API to separate the steps of producing
a diff descriptions from formatting the diff.  This will allow
us to share diff output code with the various diff creation
scenarios and will allow us to implement rename detection as
an optional pass that can be run on a diff list.

This gets the basic plumbing in place for git_diff_blob.
There is a known issue where additional parameters like
the number of lines of context to display on the diff
are not working correctly (which leads one of the new
unit tests to fail).

This cleans up the various GCC compiler warnings with the
xdiff code that was copied in.

Since casting to void works to eliminate errors with unused
parameters on all platforms, avoid the various special cases.
Over time, it will make sense to eliminate the GIT_UNUSED
macro completely and just have GIT_UNUSED_ARG.

This is the initial import of the xdiff code (LGPL) from
core git as of rev f349b562086e2b7595d8a977d2734ab2ef9e71ef

This is a test that should replicate an issue that Peff
is setting with git attributes.  But the test doesn't fail.

Filters, yo

This adds some initial unit tests for file filtering and fixes
some simple bugs in filter application.

The point of having `GIT_ATTR_TRUE` and `GIT_ATTR_FALSE` macros is to be
able to change the way that true and false values are stored inside of
the returned gitattributes value pointer.

However, if these macros are implemented as a simple rename for the
`git_attr__true` pointer, they will always be used with the `==`
operator, and hence we cannot really change the implementation to any
other way that doesn't imply using special pointer values and comparing
them!

We need to do the same thing that core Git does, which is using a
function macro. With `GIT_ATTR_TRUE(attr)`, we can change
internally the way that these values are stored to anything we want.

This commit does that, and rewrites a large chunk of the attributes test
suite to remove duplicated code for expected attributes, and to
properly test the function macro behavior instead of comparing
pointers.

Comments soothe my soul.

Sane API for real-world usage.

Friendlier revision walking

It's not unusual to want the walker to act on HEAD, so add a
convencience function for the case that the user doesn't already have
a resolved HEAD reference.