Assorted CMake fixups

cygwin also doesn't have qsort_r

An empty string is not a valid number, and some shells complain.

Check instead if $COVERITY is non-empty, which is a common convention
and what we're doing anyway.

If it's not documented, it doesn't show up in the docs (and we really
should document, anyway).

It reportedly works. It does not however work when cross-compiling on
Travis, so let's disable it there.

This fixes #2311.

This fixes #2118

The point of this phase is to know what we have and not. Show the user a
clear indication of what we have.

MidnightBSD may also not have strnlen

Make the build CMake 2.6 compatible

Keep the deflate buffer from ballooning up

Make sure we set the output parameter to a value.

When deflating data, we might need to grow the buffer. Currently we
add a guess on top of the currently-allocated buffer size.

When we re-use the buffer, it already has some memory allocated; adding
to that means that we always grow the buffer regardless of how much we
need to use.

Instead, increase on top of the currently-used size. This still leaves
us with the allocated size of the largest object we compress, but it's a
minor pain compared to unbounded growth.

This fixes #2285.

Plug some leaks

Fire progress and update tips callbacks also for pushes.

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.