Create a diff patch object as a replacement for iterators

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

Tags: teach git_tag_list not to include the 'refs/tags/' prefix

Topic/is valid name

Adjusts refs::list test (including the comments)
Adjusts objects::tags::list test

This patch changes the tag listing test helper to use a struct as input
parameter, which tells what we exactly expect.

As I don't think, we can rely on the fact that every os and every
filesystem will report the tags in the same order, I made this code
independent of the order that the tags are retrieved.

Since quite a while now, git_branch_foreach has learnt to list branches
without the 'refs/heads/' or 'refs/remotes' prefixes.
This patch teaches git_tag_list to do the same for listing tags.

Make clear that git_odb_hashfile does not use filters

Signed-off-by: Sven Strickroth <email@cs-ware.de>

There has been discussion for a while about making some set of
the `giterr_set` type functions part of the public API for code
that is implementing new backends to libgit2.  This makes the
`giterr_set_str()` and `giterr_set_oom()` functions public.

checkout: add notification callback for skipped files

git_repository_hashfile: Only close file handle if we have a valid one

Otherwise this throws an exception on MFC based systems.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Checkout, reset and others enhancements