When diff is scanning the working directory, if it finds a file
where it is not sure if the index entry matches the working dir,
it will recalculate the OID (which is pretty expensive).  This
adds a new flag to diff so that if the OID calculation finds that
the file actually has not changed (i.e. just the modified time was
altered or such), then it will refresh the stat cache in the index
so that future calls to diff will not have to check the oid again.

This reorganized the diff OID calculation to make it easier to
correctly update the stat cache during a diff once the flags to
do so are enabled.

This includes marking the path of a git_index_entry as const so
we can make a "fake" git_index_entry with a "const char *" path
and not get warnings.  I was a little surprised at how unobtrusive
this change was, but I think it's probably a good thing.

When we think the stat cache in the index seems valid and the size
or mode of a file has definitely changed, then don't bother trying
to recalculate the OID of the workdir bits to confirm that it is
modified - just accept that it is modified.

This can result in files that show as modified with no actual diff,
but the behavior actually appears to match Git on the command line.

This also includes a minor optimization to not perform a submodule
lookup on the ".git" directory itself.

See https://github.com/libgit2/libgit2/pull/2321#issuecomment-42039673
We may rollback once we found something more reliable

Make examples/status.c compile on Windows

Make ** pattern eat trailing slash

This allows "foo/**/*.html" to match "foo/file.html"

Solaris!

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.