Files in status will, be default, be sorted according to the case
insensitivity of the filesystem that we're running on.  However,
in some cases, this is not desirable.  Even on case insensitive
file systems, 'git status' at the command line will generally use
a case sensitive sort (like 'ls').  Some GUIs prefer to display a
list of file case insensitively even on case-sensitive platforms.

This adds two new flags: GIT_STATUS_OPT_SORT_CASE_SENSITIVELY
and GIT_STATUS_OPT_SORT_CASE_INSENSITIVELY that will override the
default sort order of the status output and give the user control.
This includes tests for exercising these new options and makes
the examples/status.c program emulate core Git and always use a
case sensitive sort.

When scanning untracked directories looking for non-ignored files
there was a bug where an empty directory would generate a false
error.

This adds three new public APIs for manipulating the index:

1. `git_index_add_all` is similar to `git add -A` and will add
   files in the working directory that match a pathspec to the
   index while honoring ignores, etc.
2. `git_index_remove_all` removes files from the index that match
   a pathspec.
3. `git_index_update_all` updates entries in the index based on
   the current contents of the working directory, either added
   the new information or removing the entry from the index.

Trees are always case sensitive.  The index is always case
preserving and will be case sensitive when it is turned into a
tree.  Therefore the tree and the index can and should always
be compared to one another case sensitively.

This will restore the index to case insensitive order after the
diff has been generated.

Consider this a short-term fix.  The long term fix is to have the
index always stored both case sensitively and case insensitively
(at least on platforms that sometimes require case insensitivity).

This commit reinstates some changes to git_diff__paired_foreach
that were discarded during the rebase (because the diff_output.c
file had gone away), and also adjusts the case insensitively
logic slightly to hopefully deal with either mismatched icase
diffs and other case insensitivity scenarios.

This makes diff more careful about picking the canonical path
when generating a delta so that it won't accidentally pick up a
case-mismatched path on a case-insensitive file system.  This
should make sure we use the "most accurate" case correct version
of the path (i.e. from the tree if possible, or the index if
need be).

This is a significant reorganization of the diff code to break it
into a set of more clearly distinct files and to document the new
organization.  Hopefully this will make the diff code easier to
understand and to extend.

This adds a new `git_diff_driver` object that looks of diff driver
information from the attributes and the config so that things like
function content in diff headers can be provided.  The full driver
spec is not implemented in the commit - this is focused on the
reorganization of the code and putting the driver hooks in place.

This also removes a few #includes from src/repository.h that were
overbroad, but as a result required extra #includes in a variety
of places since including src/repository.h no longer results in
pulling in the whole world.

This adds a `git__memset` routine that will not be optimized away
and updates the places where I memset() right before a free() call
to use it.