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.