Untangle git_futils_mkdir from git_futils_mkdir_ext - the latter
assumes that we own everything beneath the base, as if it were
being called with a base of the repository or working directory,
and is tailored towards checkout and ensuring that there is no
bogosity beneath the base that must be cleaned up.

This is (at best) slow and (at worst) unsafe in the larger context
of a filesystem where we do not own things and cannot do things like
unlink symlinks that are in our way.

On case insensitive filesystems, we may have files in the working
directory that case fold to a name we want to write.  Remove those
files (by default) so that we will not end up with a filename that
has the unexpected case.

Checkout can now provide performance data about the number of (some)
syscalls performed using an optional callback.

When passed the LINK_FILES flag, the recursive copy will hardlink files
instead of copying them.

The checks to see if files were out of date in the attibute cache
was wrong because the cache-breaker data wasn't getting stored
correctly.  Additionally, when the cache-breaker triggered, the
old file data was being leaked.

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.