This adds support for roughly-right tracking of submodules
(although it does not recurse into submodules to detect
internal modifications a la core git), and it adds support
for including unmodified files in diff iteration if requested.

This is a work in progress.  This adds two new sets of tests,
the issue_592 tests from @nulltoken's pull request #601 and
some new tests for submodules.  The submodule tests still have
issues where the status is not reported correctly.  That needs
to be fixed before merge.

This is an initial reimplementation of status using diff a la
the way that core git does it.

Cleaned up some other issues.

More files moved to new error handling style.

This includes a few cleanups that came up while converting
these files.

This commit introduces a could new git error classes, including
the catchall class: GITERR_INVALID which I'm using as the class
for invalid and out of range values which are detected at too low
a level of library to use a higher level classification.  For
example, an overflow error in parsing an integer or a bad letter
in parsing an OID string would generate an error in this class.

Also cleaned up some previously converted code that still had
little things to polish.

Fix windows network paths

Fix libgit2/libgit2sharp#125

Update odb code to new error handling

This converts blob.c, fileops.c, and all of the win32 files.
Also, various minor cleanups throughout the code.  Plus, in
testing the win32 build, I cleaned up a bunch (although not
all) of the warnings with the 64-bit build.

This continues to add other files to the new error handling
style.  I think the only real concerns here are that there are
a couple of error return cases that I have converted to asserts,
but I think that it was the correct thing to do given the new
error style.

This converts the map validation function into a macro, tweaks
the GITERR_OS system error automatic appending, and adds a
tentative new error access API and some quick unit tests for
both the old and new error APIs.

Forgot to add this file in the previous commit

This migrates odb.c, odb_loose.c, odb_pack.c and pack.c to
the new style of error handling.  Also got the unix and win32
versions of map.c.  There are some minor changes to other
files but no others were completely converted.

This also contains an update to filebuf so that a zeroed out
filebuf will not think that the fd (== 0) is actually open
(and inadvertently call close() on fd 0 if cleaned up).

Lastly, this was built and tested on win32 and contains a
bunch of fixes for the win32 build which was pretty broken.

Includes:

	- Proper error reporting when encountering syntax errors in a
	config file (file, line number, column).

	- Rewritten `config_write`, now with 99% less goto-spaghetti

	- Error state in `git_filebuf`: filebuf write functions no longer
	need to be checked for error returns. If any of the writes performed
	on a buffer fail, the last call to `git_filebuf_commit` or
	`git_filebuf_hash` will fail accordingly and set the appropiate error
	message. Baller!

Migrating diff to new error handling

This resolves the comments on pull request #590

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.