Update showindex example

I find the showindex example to be pretty useful on occasion, but
there were are couple of output tweaks I wanted, plus I wanted the
ability to specify a path to an actual index file instead of having
to open the whole repository.  This makes those changes and expands
the example slightly.

Alternative fix for iterator memory leak

Fixed compilation with =>VS2010

Starting with VS2010 MS ships a stdint.h.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

When normalizing a reference name, if there is an error because
the name is invalid, then the memory allocated for storing the
name could be leaked if the caller was not careful and assumed
that the error return code meant that no allocation had occurred.

This fixes that by explicitly deallocating the reference name
buffer if there is an error in normalizing the name.

An earlier change to `git_diff_from_iterators` introduced a
memory leak where the allocated spoolandsort iterator was not
returned to the caller and thus not freed.

One proposal changes all iterator APIs to use git_iterator** so
we can reallocate the iterator at will, but that seems unexpected.
This commit makes it so that an iterator can be changed in place.
The callbacks are isolated in a separate structure and a pointer
to that structure can be reassigned by the spoolandsort extension.

This means that spoolandsort doesn't create a new iterator; it
just allocates a new block of callbacks (along with space for its
own extra data) and swaps that into the iterator.

Additionally, since spoolandsort is only needed to switch the
case sensitivity of an iterator, this simplifies the API to only
take the ignore_case boolean and to be a no-op if the iterator
already matches the requested case sensitivity.

Teach UNC paths to git_path_dirname_r()

Fix libgit2/libgit2sharp#256

odb: check if object exists before writing

Update the procondition of git_odb_backend::write.
It may now be assumed that the object has already been hashed.

Mark travis build as failed if any test fails

fetchhead reading/iterating

netops: on SSL teardown only send shutdown alert

Make `goto cleanup` more consistent

RFC: Some cleanups/features in CMakeLists.txt

Fix some leaks in tests

Also adds some asserts.

... so we can clean up correctly.

Instead tell MSVC to group the source files by directory.

- Also document the -DSTDCALL even better.