add-log-keep-changes-together is a variable defined in `add-log.el'.
Its value is

If non-nil, normally keep day's log entries for one file together.

Log entries for a given file made with M-x add-change-log-entry or
C-x 4 a will only be added to others for that file made
today if this variable is non-nil or that file comes first in today's
entries. Otherwise another entry for that file will be started. An
original log:

* foo (...): ...
* bar (...): change 1

in the latter case, C-x 4 a in a buffer visiting `bar', yields:

* bar (...): -!-
* foo (...): ...
* bar (...): change 1

and in the former:

* foo (...): ...
* bar (...): change 1
(...): -!-

The NEW-ENTRY arg to `add-change-log-entry' can override the effect of
this variable.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 20.3 of Emacs.