This adds in missing calls to `git_buf_sanitize` and fixes a
number of places where `git_buf` APIs could inadvertently write
NUL terminator bytes into invalid buffers.  This also changes the
behavior of `git_buf_sanitize` to NUL terminate a buffer if it can
and of `git_buf_shorten` to do nothing if it can.

Adds tests of filtering code with zeroed (i.e. unsanitized) buffer
which was previously triggering a segfault.

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 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.

I wrote this stuff a while ago and forgot to write tests.  Wanted
to do so now to wrap up the PR and immediately found problems.

The base for the relative urls is determined as follows, with descending
priority:

- remote url of HEAD's remote tracking branch
- remote "origin"
- workdir

This follows git.git behaviour

Wrote tests that try adding, removing, and updating the name of
submodules which showed a number of problems with how we account
for changes when incrementally updating the submodule info.  Most
of these issues didn't exist before because reloading would always
blow away the old submodule data.

This improvement the management of the lock around submodule cache
updates slightly, using the lock to make sure that foreach can
safely make a snapshot of all existing submodules and making sure
that git_submodule_add_setup also grabs a lock before inserting
the new submodule.  Cache initialization / refresh should already
have been holding the lock correctly as it adds submodules.

This makes it so that git_submodule_reload_all will actually only
reload changed items unless the `force` flag is used.

When forcing cache flushes or reload, etc., it is easier to keep
track of intent using enums instead of plain bools.  Also, this
fixes a bug where the cache was not being properly refreshes by
a git_submodule_reload_all.