This fixes the missing readdir_r from win32 and fixes other
platforms to always use the reentrant readdir_r form for reading
directory contents.

Fix -Wuninitialized warning

Signed-off-by: schu <schu-github@schulog.org>

Uniform iterators for trees, index, and workdir

This update addresses all of the feedback in pull request #570.

The biggest change was to create actual linked list stacks for
storing the tree and workdir iterator state.  This cleaned up
the code a ton.  Additionally, all of the static functions had
their 'git_' prefix removed, and a lot of other unnecessary
changes were removed from the original patch.

This makes two changes to iterator behavior: first, advance
can optionally do the work of returning the new current value.
This is such a common pattern that it really cleans up usage.

Second, for workdir iterators, this removes automatically
iterating into directories.  That seemed like a good idea,
but when an entirely new directory hierarchy is introduced
into the workdir, there is no reason to iterate into it if
there are no corresponding entries in the tree/index that it
is being compared to.

This second change actually wasn't a lot of code because not
descending into directories was already the behavior for
ignored directories.  This just extends that to all directories.

Add config multivar support

Export parse_tag_buffer as git_tag__parse_buffer.

This create a new git_iterator type of object that provides a
uniform interface for iterating over the index, an arbitrary
tree, or the working directory of a repository.

As part of this, git ignore support was extended to support
push and pop of directory-based ignore files as the working
directory is being traversed (so the array of ignores does
not have to be recreated at each directory during traveral).

There are a number of other small utility functions in buffer,
path, vector, and fileops that are included in this patch
that made the iterator implementation cleaner.

Fix copy-paste error

Remotes improvements

Allow setting the fetch and push refspecs, which is useful for
creating new refspecs.

Windows doesn't support POSIX regex, so we need to include it
ourselves. The sources come from git, which in turn took them from
gawk.

Move the configuration to use a multimap instead of a list. This
commit doesn't provide any functional changes but changes the support
structures.

notes: fix assert

Hopefully fix issue "Don't sleep and code" - #558.

Signed-off-by: schu <schu-github@schulog.org>

Notes API

This commit adds basic git notes support to libgit2, namely:

* git_note_read
* git_note_message
* git_note_oid
* git_note_create
* git_note_remove

In the long run, we probably want to provide some convenience callback
mechanism for merging and moving (filter-branch) notes.

Signed-off-by: schu <schu-github@schulog.org>

This is legacy compat stuff for when `deflateBound` is not defined, but
we're not embedding zlib and that function is always available. Kill
that with fire.

git__ishex allows to check if a string is a hexadecimal representation.

Signed-off-by: schu <schu-github@schulog.org>

git_commit_create is supposed to update the given reference
"update_ref", but segfaulted in case of a yet to be born
reference. Fix it.

Signed-off-by: schu <schu-github@schulog.org>

Update Copyright header

Signed-off-by: schu <schu-github@schulog.org>

Fix "clang warning: expression result unused"

tests-clar: fix warning sign-compare

Signed-off-by: schu <schu-github@schulog.org>