magit-blame: remove unnecessary code
magit-blame: use window-{start,end} for range

Subtract 1 from `window-end' because there is disagreement
between Emacs and Git on how long the file is.
magit-blame{,-process-sentinel}: make file buffer current
magit-diff-wash-diff: detect more submodule states
magit-insert-tags: only if there are tags

otherwise no longer insert an empty section.