According to the reference the git_checkout_tree and git_checkout_head
functions should accept NULL in the opts field

This was broken since the opts field was dereferenced and thus lead to a
crash.

When no index file exists and a baseline is not explicitly provided, use
an empty baseline instead of trying to load `HEAD`.

When a file exists on disk and we're checking out a file that differs
in executableness, remove the old file.  This allows us to recreate the
new file with p_open, which will take the new mode into account and
handle setting the umask properly.

Remove any notion of chmod'ing existing files, since it is now handled
by the aforementioned removal and was incorrect, as it did not take
umask into account.

We do not error on "merge conflicts"; on the contrary, merge conflicts
are a normal part of merging.  We only error on "checkout conflicts",
where a change exists in the index or the working directory that would
otherwise be overwritten by performing the checkout.

This *may* happen during merge (after the production of the new index
that we're going to checkout) but it could happen during any checkout.