This corrects the order of operations in git reset so that the
checkout to reset the working directory content is done before
the HEAD is moved.  This allows us to use the HEAD and the index
content to know what files can / should safely be reset.

Unfortunately, there are still some cases where the behavior of
this revision differs from core git.  Notable, a file which has
been added to the index but is not present in the HEAD is
considered to be tracked by core git (and thus removable by a
reset command) whereas since this loads the target state into
the index prior to resetting, it will consider such a file to be
untracked and won't touch it.  That is a larger fix that I'll
defer to a future commit.