And decrease extra reload checks of config data.

Introduce core.safecrlf handling

This adds a basic test of doing simultaneous diffs on multiple
threads and adds basic locking for the attr file cache because
that was the immediate problem that arose from these tests.

This takes the old submodule cache which was just a git_strmap
and makes a real git_submodule_cache object that can contain other
things like a lock and timestamp-ish data to control refreshing of
submodule info.

This doesn't actual do string precompose but it puts the hooks in
place into the iterators and the git_path_dirload function so that
the actual precompose work is ready to go.

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.

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...