Support more diff config options and use the config cache more

This makes diff use the cvar cache for config options where
possible, and also adds support for a number of other config
options to diff including "diff.context", "diff.ignoreSubmodules",
"diff.noprefix", "diff.mnemonicprefix", and "core.abbrev".

To make this natural, this involved a rearrangement of the code
that allocates the diff object vs. the code that initializes it
based on the combination of options passed in by the user and
read from the config.

This commit includes tests for most of these new options as well.

This is a conservative change, but it seemed like the only safe
thing to do -- i.e. clear the cvar cache when a config gets set.

This converts many of the config lookups that are done around the
library to use the repository config cache.  This was everything I
could find that wasn't part of diff (which requires a larger fix).

This adds a bunch of additional config values to the repository
config value cache and makes it easier to add a simple boolean
config without creating enum values for each possible setting.

Also, this fixes a bug in git_config_refresh where the config
cache was not being cleared which could lead to potential
incorrect values.

The work to start using the new cached configs will come in the
next couple of commits...

When case insensitive tree iterators were added, we started reading
the case sensitivity of the index to decide if the tree should be
case sensitive.  This is good for index-to-tree comparisons, but
for tree-to-tree comparisons, we should really default to doing a
case sensitive comparison unless the user really wants otherwise.

Remote refspecs minor fixes

64 bit atomic operations and shared cache memory usage

Added function to insert commit into pack

Make refcounting atomic

New caching

Rename git_packfile_check to git_packfile_alloc since it is now
being used more in that capacity.  Fix the various places that use
it.  Consolidate some repeated code in odb_pack.c related to the
allocation of a new pack_backend.

The indexer was creating a packfile object separately from the
code in pack.c which was a problem since I put a call to
git_mutex_init into just pack.c.  This commit updates the pack
function for creating a new pack object (i.e. git_packfile_check())
so that it can be used in both places and then makes indexer.c
use the shared initialization routine.

There are also a few minor formatting and warning message fixes.

This removes the lock from the repository object and changes the
internals to use the new atomic git__compare_and_swap to update
the _odb, _config, _index, and _refdb variables in a threadsafe
manner.

This builds on the earlier thread safety work to make it so that
setting the odb, index, refdb, or config for a repository is done
in a threadsafe manner with minimized locking time.  This is done
by adding a lock to the repository object and using it to guard
the assignment of the above listed pointers.  The lock is only
held to assign the pointer value.

This also contains some minor fixes to the other work with pack
files to reduce the time that locks are being held to and fix an
apparently memory leak.