magit-insert-child-count: massive speed-up

Previously we used `looking-back' to check whether a section's heading
ended with ":" indicating that a child count should be inserted.  That
had a devastating effect on performance.  We now use `char-before'.
And generally avoid all unnecessary work.
Merge branch 'n/turbo' into next
magit-blame: run git-blame in top directory
magit-blame-format-heading: remove leftover invisible property

Don't set the `invisible` property, that is a leftover from an earlier
attempt of switching between the two separator styles.  This leftover
caused the headings to be invisible, but for some unknown reasons only
in some buffers.
bind j instead of M-g to magit-jump-to-diffstats

Do so in the two relevant keymaps `magit-commit-mode-map' and
`magit-diff-mode-map'.  In other maps `j' is bound to the related
`magit-section-jump-map'.
Merge branch 'master' into next

Conflicts:
	magit.el
magit.el: explicitly set coding system to utf-8

Unlike the other libraries `magit.el' contains some non-ascii
characters.  Usually Emacs used utf-8 if nothing else is specified
but for some language environments this does not seem to be the case.
Merge branch 'master' into next
bind `C-x a' instead of `L' to magit-add-change-log-entry

`L' is now bound to `magit-log-toggle-margin', but this was
still overwritten by the older binding of the same key to
`magit-add-change-log-entry'.  Since `C-x 4 a` has been bound
to `magit-add-change-log-entry-other-window' for a long time
bind `C-x a` to the "same window" variant.