This takes the `--stat` and related example options in the example
diff.c program and converts them to use the `git_diff_get_stats`
API which nicely formats stats for you.

I went to add bar-graph scaling to the stats formatter and noticed
that the `git_diff_stats` structure was holding on to all of the
`git_patch` objects.  Unfortunately, each of these objects keeps
the full text of the diff in memory, so this is very expensive.  I
ended up modifying `git_diff_stats` to keep just the data that it
needs to keep and allowed it to release the patches.  Then, I added
width scaling to the output on top of that.

In making the diff example program match 'git diff' output, I ended
up removing an newline from the sumamry output which I then had to
compensate for in the email formatting to match the expectations.

Lastly, I went through and refactored the tests to use a couple of
helper functions and reduce the overall amount of code there.

I was playing with "git diff-index" and wanted to be able to
emulate that behavior a little more closely with the diff example.

Also, I wanted to play with running `git_diff_tree_to_workdir`
directly even though core Git doesn't exactly have the equivalent,
so I added a command line option for that and tweaked some other
things in the example code.

This changes a minor output thing in that the "raw" print helper
function will no longer add ellipses (...) if the OID is not
actually abbreviated.

A few niceties for binding authors

Allow the credentials callback to return GIT_PASSTHROUGH to make the
transports code behave as though none was set.

This should make it easier for bindings to behave closer to the C code
when there is no credentials callback set at their level.

This should make it easier for bindings to dynamically override their
own callbacks.

Fix some issues from the last Coverity scan

Fix reset for staged deletes