Update diff handling of "untracked" directories

This includes more tests for various scenarios when diff includes
an untracked directory in the workdir with contents either ignored
or not.

When diff encounters an untracked directory, there was a shortcut
that it took which is not compatible with core git.  This makes
the default behavior no longer take that shortcut and instead look
inside the untracked directory to see if there are any untracked
files within it.  If there are not, then the directory is treated
as an ignore directory instead of an untracked directory.  This
has implications for the git_status APIs.

In preparation for more changes to the internal diff logic, it
seemed wise to split the very large git_diff__from_iterators into
separate functions that handle the four main cases (unmatched old
item, unmatched new item, unmatched new directory, and matched
old and new items).  Hopefully this will keep the logic easier to
follow even as more cases have to be added to this code.

Remove most inlines from the public API

Add refspec list accessors

Catch issue in config set with no config file

This prevents a segfault when setting a value in the config of a
repository that doesn't have a config file.

Bring back a way of acessing the git_refspec* from a remote.

Closes #1514

Removed useless prototype and renamed object typecast functions
declaration macro.

This removes the GIT_INLINE versions of the simple git_object
accessors and standardizes them with a helper macro in src/object.h
to build the function bodies.

dwim shorthand refspecs for fetch

don't use uninitialized struct stat in win32

cmake 2.6 parser bug workaround

Add a new git_oid_strcmp that compares a string OID with a hex
oid for sort order, and then reimplement git_oid_streq using it.
This actually should speed up git_oid_streq because it only reads
as far into the string as it needs to, whereas previously it would
convert the whole string into an OID and then use git_oid_cmp.

git_oid_ncmp was making some assumptions about the length of
the data - this shifts the check to the top of the loop so it
will work more robustly, limits the max, and adds some tests
to verify the functionality.

Fix push for delete-only and add travis rules to test it

Create a test repository in the VM and set up git-daemon so we can
use it to test the push code.

For update and create commands where all the objects are known to
exist in the remote, we must send an empty packfile. However, if all
we issue are delete commands, no packfile must be sent.

Take this into consideration for push.

As git allows you to store shorthand refspecs in the configuration, we
need to do this ourselves.

Relax the ONELEVEL ref naming rules so the refspec parsing code can
ask for 'master' to be considered valid.

git_atomic_ssize for 64-bit atomics only on 64-bit platforms

Support more diff config options and use the config cache more