The adds a test for the submodule diff capabilities and then
fixes a few bugs with how the output is generated.  It improves
the accuracy of OIDs in the diff delta object and makes the
submodule output more closely mirror the OIDs that will be used
by core git.

There are a few cases where diff should leave directories in
the diff list if we want to match core git, such as when the
directory contains a .git dir.  That feature was lost when I
introduced some of the new submodule handling.

This restores that and then fixes a couple of related to diff
output that are triggered by having diffs with directories in
them.

Also, this adds a new flag that can be passed to diff if you
want diff output to actually include the file content of any
untracked files.

Diff: Show submodule diff

Fix a bug where ignorecase wasn't applied to ignores

The reference is only needed inside the function. We mistakenly
increased the reference counter causing the ODB not to get freed and
leaking descriptors.

Storing flushes in the refs vector doesn't let us recognize when the
remote is empty, as we'd always introduce at least one element into
it. These flushes aren't necessary, so we can simply ignore them.

We don't have anything useful that we could do with that oid anyway (We
need to query the submodule for the HEAD commit instead).

Without this, the following code creates the error "Failed to read
descriptor: Is a directory" when run against the submod2 test-case:

    const char* oidstr = "873585b94bdeabccea991ea5e3ec1a277895b698";
    git_tree* tree = resolve_commit_oid_to_tree(g_repo, oidstr);
    git_diff_list* diff = NULL;
    cl_assert(tree);
    cl_git_pass(git_diff_workdir_to_tree(g_repo, NULL, tree, &diff));

1. teach diff.c:maybe_modified to query git_submodule_status for the
   modification state of a submodule. According to the
   git_submodule_status docs, it will filter for to-ignore states
   already.

2. teach diff_output.c:get_workdir_content to check the submodule status
   again and create a line like:

      Subproject commit <SHA-1>\n
   or
      Subproject comimt <SHA-1>-dirty\n

   like git.git does.

diff_output.c:get_blob_content used to try to read the submodule commit
as a blob in the superproject's odb. Of course it cannot find it and
errors out with GIT_ENOTFOUND, implcitly terminating the whole diff
output.

This patch teaches it to create a text that describes the submodule
instead. The text looks like:

	Subproject commit <SHA1>\n

which is what git.git does, too.

ODB: re-load packfiles on failed lookup

Support for the core.ignorecase flag

Win64 cleanups

Include tag

Also honor remote.$name.tagopt = --tags.

The local function works for simple cases, but we shouldn't reinvent
the wheel just for us.

Together with include-tag, this make us behave more like git. After a
fetch, try to create any tags the remote told us about for which we
have objects locally.

This tells the remote to send us any tags that point to objects that
we are downloading.

Indicate whether the error comes from the ref already existing or
elsewhere. We always perform the check and this lets the user write
more concise code.

The latter shouldn't be exposed and isn't used, git_refspec__parse
supersedes it.

Fix a leak in the refspec tests while we're at it.

Fix error hashing empty file.

revwalk: fix off-by-one error

Create a diff patch object as a replacement for iterators

Fixes #921.

There are a lot of places where the diff API gives the user access
to internal data structures and many of these were being exposed
through non-const pointers.  This replaces them all with const
pointers for any object that the user can access but is still
owned internally to the git_diff_list or git_diff_patch objects.

This will probably break some bindings...  Sorry!

This fixes all the bugs in the new diff patch code.  The only
really interesting one is that when we merge two diffs, we now
have to actually exclude diff delta records that are not supposed
to be tracked, as opposed to before where they could be included
because they would be skipped silently by `git_diff_foreach()`.
Other than that, there are just minor errors.

Replacing the `git_iterator` object, this creates a simple API
for accessing the "patch" for any file pair in a diff list and
then gives indexed access to the hunks in the patch and the lines
in the hunk.  This is the initial implementation of this revised
API - it is still broken, but at least builds cleanly.

Allow diff deltas to be accessed by index and make patch generation
explicit with hunk and line access by index as well.

Make giterr_set_str() and giterr_set_oom() public APIs