slight inconsistency with fixups cwd during mr checkout

[Adam Spiers]

If I mr checkout a repo which has fixups defined, they are run from
the parent of $MR_REPO, whereas if the fixups are run via 'mr fixups'
or 'mr update', they are run from inside $MR_REPO.  This means that
fixups need an extra 'cd' at the top if they need to run from inside
the repo, which is pretty common, e.g.:

    fixups =
        cd "$MR_REPO"
        make install

It would be nice if mr did the chdir in the checkout case too, to make
things more consistent.