This makes the index iterator honor the GIT_ITERATOR_IGNORE_CASE
and GIT_ITERATOR_DONT_IGNORE_CASE flags without modifying the
index data itself.  To take advantage of this, I had to export a
number of the internal index entry comparison functions.  I also
wrote some new tests to exercise the capability.

We need this from util.h and posix.h, but the latter includes common.h
which includes util.h, which means p_strlen is not defined by the time
we get to git__strndup().

Split the definition on p_strlen() off into its own header so we can use
it in util.h.

The standard library provides a very nice strnlen function, which knows
to use SSE, let's not reimplement it ourselves.

This adds the basics of progress reporting during push. While progress
for all aspects of a push operation are not reported with this change,
it lays the foundation to add these later. Push progress reporting
can be improved in the future - and consumers of the API should
just get more accurate information at that point.

The main areas where this is lacking are:

1) packbuilding progress: does not report progress during deltafication,
   as this involves coordinating progress from multiple threads.

2) network progress: reports progress as objects and bytes are going
   to be written to the subtransport (instead of as client gets
   confirmation that they have been received by the server) and leaves
   out some of the bytes that are transfered as part of the push protocol.
   Basically, this reports the pack bytes that are written to the
   subtransport. It does not report the bytes sent on the wire that
   are received by the server. This should be a good estimate of
   progress (and an improvement over no progress).