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...

The goal of this work is to expose the search logic for "global",
"system", and "xdg" files through the git_libgit2_opts() interface.

Behind the scenes, I changed the logic for finding files to have a
notion of a git_strarray that represents a search path and to store
a separate search path for each of the three tiers of config file.
For each tier, I implemented a function to initialize it to default
values (generally based on environment variables), and then general
interfaces to get it, set it, reset it, and prepend new directories
to it.

Next, I exposed these interfaces through the git_libgit2_opts
interface, reusing the GIT_CONFIG_LEVEL_SYSTEM, etc., constants
for the user to control which search path they were modifying.
There are alternative designs for the opts interface / argument
ordering, so I'm putting this phase out for discussion.

Additionally, I ended up doing a little bit of clean up regarding
attr.h and attr_file.h, adding a new attrcache.h so the other two
files wouldn't have to be included in so many places.

Often `git_odb_read_header` will "fail" and have to read the
entire object into memory instead of just the header.  When this
happens, the object is loaded and then disposed of immediately,
which makes it difficult to efficiently use the header information
to decide if the object should be loaded (since attempting to do
so will often result in loading the object twice).

This commit takes the existing code and reorganizes it to have
two new functions:

- `git_odb__read_header_or_object` which acts just like the old
  read header function except that it returns the object, too, if
  it was forced to load the whole thing.  It then becomes the
  callers responsibility to free the `git_odb_object`.
- `git_object__from_odb_object` which was extracted from the old
  `git_object_lookup` and creates a subclass of `git_object` from
  an existing `git_odb_object` (separating the ODB lookup from the
  `git_object` creation).  This allows you to use the first header
  reading function efficiently without instantiating the
  `git_odb_object` twice.

There is no net change to the behavior of any of the existing
functions, but this allows internal code to tap into the ODB
lookup and object creation to be more efficient.