Clarify the `git_atomic` type and functions now that we have a 64 bit
version as well (`git_atomic64`).

Move to the `git_error` name in the internal API for error-related
functions.

Our thread policies state that we cannot re-use the `git_repository`
across threads.  Our tests cannot deviate from that.

The threads::diff test suite has a static variable `_retries`, which is
used on Windows platforms only. As it is unused on other systems, the
compiler throws a warning there. Fix the warning by wrapping the
declaration in an ifdef.

POSIX emulation retries should be configurable so that tests can disable
them.  In particular, maniacally threading tests may end up trying to
open locked files and need retries, which will slow continuous
integration tests significantly.

Move the definition of git_thread_yield() to the test which needs it and
add the correct definition for it for FreeBSD and derivatives.

Original patch adding FreeBSD and derivatives by @jacquesg.

In the threading tests, I was still seeing a race condition where
the same item could end up being inserted multiple times into the
index.  Preserving the sorted-ness of the index outside of the
`index_insert` call fixes the issue.

This is a big refactoring of the attribute file cache to be a bit
simpler which in turn makes it easier to enforce a lock around any
updates to the cache so that it can be used in a threaded env.
Tons of changes to the attributes and ignores code.

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 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.