Update cl_git_pass to return more info

This adds a failure reporting function that is called by
cl_git_pass which captures the actual error return code and
the error message if available in the failure report.
Clear error to avoid leaving invalid error behind
missing error message is confusing
Add failing test to demonstrate wrong checkout behaviour
Failing test on git_checkout_tree when removing directories
Reorder operations in git reset

This corrects the order of operations in git reset so that the
checkout to reset the working directory content is done before
the HEAD is moved.  This allows us to use the HEAD and the index
content to know what files can / should safely be reset.

Unfortunately, there are still some cases where the behavior of
this revision differs from core git.  Notable, a file which has
been added to the index but is not present in the HEAD is
considered to be tracked by core git (and thus removable by a
reset command) whereas since this loads the target state into
the index prior to resetting, it will consider such a file to be
untracked and won't touch it.  That is a larger fix that I'll
defer to a future commit.
Rework checkout internals (again)

I've tried to map out the detailed behaviors of checkout and make
sure that we're handling the various cases correctly, along with
providing options to allow us to emulate "git checkout" and "git
checkout-index" with the various flags.  I've thrown away flags
in the checkout API that seemed like clutter and added some new
ones.  Also, I've converted the conflict callback to a general
notification callback so we can emulate "git checkout" output and
display "dirty" files.

As of this commit, the new behavior is not working 100% but some
of that is probably baked into tests that are not testing the
right thing.  This is a decent snapshot point, I think, along the
way to getting the update done.
More checkout improvements

This flips checkout back to be driven off the changes between
the baseline and the target trees.  This reinstates the complex
code for tracking the contents of the working directory, but
overall, I think the resulting logic is easier to follow.
Add index updating to checkout

Make checkout update entries in the index for all files that are
updated and/or removed, unless flag GIT_CHECKOUT_DONT_UPDATE_INDEX
is given.  To do this, iterators were extended to allow a little
more introspection into the index being iterated over, etc.
Add showindex option for bare index

I use the showindex example for debugging a fair amount.  This
adds two pieces - the ability to show based on a path directly
to an index file and some addition entry data output.
Make git_oid_tostr use out buffer for NULL oid

Previously a NULL oid was handled like an empty buffer and
returned a status empty string.  This makes git_oid_tostr()
set the output buffer to the empty string instead.
Improve error propagation in stash

Stash was sometimes obscuring the actual error code, replacing it
with a -1 when there was more descriptive value.  This updates
stash to preserve the original error code more reliably along
with a variety of other error handling tweaks.

I believe this is an improvement, but arguably, preserving the
underlying error code may result in values that are harder to
interpret by the caller who does not understand the internals.
Discussion is welcome!