This makes the lock management on the index a little bit broader,
having a number of routines hold the lock across looking up the
item to be modified and actually making the modification.  Still
not true thread safety, but more pure index modifications are now
safe which allows the simple cases (such as starting up a diff
while index modifications are underway) safe enough to get the
snapshot without hitting allocation problems.

As part of this, I simplified the allocation of index entries to
use a flex array and just put the path at the end of the index
entry.  This makes every entry self-contained and makes it a
little easier to feel sure that pointers to strings aren't
being accidentally copied and freed while other references are
still being held.

This loads SRWLock APIs at runtime and in their absence (i.e. on
Windows before Vista) falls back on a regular CRITICAL_SECTION
that will not permit concurrent readers.

This makes libgit2 require Windows Vista or newer if it is going
to be compiled with the THREADSAFE option

This is the first use we have of pthread_rwlock_t in libgit2.
Hopefully it won't cause any serious portability problems.

This makes git__swap use the __sync_lock_test_and_set primitive
with GCC and the InterlockedExchangePointer primitive with MSVC.
Previously is used compare_and_swap in a way that was probably
unintuitive for most thinking (i.e. it could fail to swap in the
value if another thread raced in).  Now it will always succeed
and the last thread to run in a race will win instead of the
first thread.

This also fixes up a little confusion between volatile void **
and void * volatile * that came up with the Win32 compiler.

This is a significant reorganization of the diff code to break it
into a set of more clearly distinct files and to document the new
organization.  Hopefully this will make the diff code easier to
understand and to extend.

This adds a new `git_diff_driver` object that looks of diff driver
information from the attributes and the config so that things like
function content in diff headers can be provided.  The full driver
spec is not implemented in the commit - this is focused on the
reorganization of the code and putting the driver hooks in place.

This also removes a few #includes from src/repository.h that were
overbroad, but as a result required extra #includes in a variety
of places since including src/repository.h no longer results in
pulling in the whole world.