This adds ~/ prefix expansion for the value of core.attributesfile
and core.excludesfile, plus it fixes the fact that the attributes
cache was holding on to the string data from the config for a long
time (instead of making its own strdup) which could have caused a
problem if the config was refreshed.  Adds a test for the new
expansion capability.

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.

This is designed to fix libgit2sharp #350 where if .gitignore is
a directory we abort all operations that process ignores instead
of just skipping it as core git does.

Also added test that fails without this change and passes with it.