The goal of this work is to rewrite git_status_file to use the
same underlying code as git_status_foreach.

This is done in 3 phases:

1. Extend iterators to allow ranged iteration with start and
   end prefixes for the range of file names to be covered.
2. Improve diff so that when there is a pathspec and there is
   a common non-wildcard prefix of the pathspec, it will use
   ranged iterators to minimize excess iteration.
3. Rewrite git_status_file to call git_status_foreach_ext
   with a pathspec that covers just the one file being checked.

Since ranged iterators underlie the status & diff implementation,
this is actually fairly efficient.  The workdir iterator does
end up loading the contents of all the directories down to the
single file, which should ideally be avoided, but it is pretty
good.

When a repo is first created, there is no HEAD yet and attempting
to diff files in the index was showing nothing because a tree
iterator could not be constructed.  This adds an "empty" iterator
and falls back on that when the head cannot be looked up.

This is a major reorganization of the diff code.  This changes
the diff functions to use the iterators for traversing the
content.  This allowed a lot of code to be simplified.  Also,
this moved the functions relating to outputting a diff into a
new file (diff_output.c).

This includes a number of other changes - adding utility
functions, extending iterators, etc. plus more tests for the
diff code.  This also takes the example diff.c program much
further in terms of emulating git-diff command line options.

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.

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.