Add view file at commit from magit-blame-mode.

magit-blame-view-file-at-commit shows the current file as it was at commit
from current position.

v is bound to magit-blame-view-file-at-commit.

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>