magit-diff-section-for-diffstat: new (kludge) function

Add the new function `magit-diff-section-for-diffstat' to replace
`magit-goto-diff-section-at-file'.  The latter did more than its only
caller, `magit-ediff' needed, resulting in needless relocation of
`point'.

The new function is a kludge, and will be removed once the functions
used for finding particular sections has been improved.