Improve test coverage of status with different core.precomposeunicode settings

Improve stash and checkout for ignored + untracked items

Merge checkout strategy

and make tests empty on platforms without iconv support.

There is an interesting difference with core Git here, though.
Because libgit2 will do rename detection with the working directory,
in the last case where the HEAD and the working directory both
have the decomposed data and the index has the composed data, we
generate a single status record with two renames whereas Git will
generate one rename (head to index) and one untracked file.

When considering status of untracked directories, if we find an
explicitly ignored item, even if it is a directory, treat the
parent as an IGNORED item.  It was accidentally being treated as
an EMPTY item because we were not looking into the ignored subdir.

fetchhead: deal with quotes in branch names

The current FETCH_HEAD parsing code assumes that a quote must end the
branch name. Git however allows for quotes as part of a branch name,
which causes us to consider the FETCH_HEAD file as invalid.

Instead of searching for a single quote char, search for a quote char
followed by SP, which is not a valid part of a ref name.

In the iterator, distinguish between ignores and empty directories
so that diff and status can ignore empty directories, but checkout
and stash can treat them as untracked items.

patch: emit deflated binary patches (optionally)

Win32: UTF-8 <-> WCHAR conversion overhaul

Some doc and examples/diff.c changes

When diff finds an untracked directory, it emulates Git behavior
by looking inside the directory to see if there are any untracked
items inside it. If there are only ignored items inside the dir,
then diff considers it ignored, even if there is no direct ignore
rule for it.

Checkout was not copying this behavior - when it found an untracked
directory, it just treated it as untracked.  Unfortunately, when
combined with GIT_CHECKOUT_REMOVE_UNTRACKED, this made is seem that
checkout (and stash, which uses checkout) was removing ignored
items when you had only asked it to remove untracked ones.

This commit moves the logic for advancing past an untracked dir
while scanning for non-ignored items into an iterator helper fn,
and uses that for both diff and checkout.

To emulate git, stash should not remove untracked git repositories
inside the parent repo, and checkout's REMOVE_UNTRACKED should
also skip over these items.

`git stash` actually prints a warning message for these items.
That should be possible with a checkout notify callback if you
wanted to, although it would require a bit of extra logic as things
are at the moment.

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

Check for compiler flag support instead of checking for a platform

Reported by coverity.

Returning an error cleared the buf, but this operation does not free the
memory associated with it. Use git_buf_free() instead.

Add support for SSH keyboard-interactive authentication

Fix several ignore and attribute file behavior bugs

Only apply LEADING_DIR pattern munging to patterns in ignore and
attribute files, not to pathspecs used to select files to operate
on.  Also, allow internal macro definitions to be evaluated before
loading all external ones (important so that external ones can
make use of internal `binary` definition).

Ignore patterns that ended with a trailing '/*' were still needing
to match against another actual '/' character in the full path.
This is not the same behavior as core Git.

Instead, we strip a trailing '/*' off of any patterns that were
matching and just take it to imply the FNM_LEADING_DIR behavior.