This adds a new diff API `git_diff_commit` which makes it easy to
generate a `git_diff` object that represents the changes in a given
commit.  It follows the core Git rules for considering changes in
a merge commit - i.e. if a file has an exact match in any parent
of the commit, then the file is considered unmodified by the merge.

This also adds a new example program "last-changed" which takes a
list of filenames and for each one displays the SHA of the last
commit that changed that file.

This allows you to use a --repeat option to run status over and
over and see how the output changes as you make local directory
changes without reopening the git_repository object each time.

Also, adds a flag to explicitly list the submodules before status.

Use git_object_short_id in examples

Rename in-memory remote to anonymous and swap url and fetch order

The order in this function is the opposite to what
create_with_fetchspec() has, so change this one, as url-then-refspec is
what git does.

As we need to break compilation and the swap doesn't do that, let's take
this opportunity to rename in-memory remotes to anonymous as that's
really what sets them apart.

`git_submodule` objects were already refcounted internally in case
the submodule name was different from the path at which it was
stored.  This makes that refcounting externally used as well, so
`git_submodule_lookup` and `git_submodule_add_setup` return an
object that requires a `git_submodule_free` when done.

Rename options structures

Add a tag example