Put post-25 ChangeLog entries into ChangeLog.3

* ChangeLog.2: Copy from emacs-25 branch.
* ChangeLog.3: New file, with changes only in master.
* Makefile.in (CHANGELOG_HISTORY_INDEX_MAX): Bump from 2 to 3.

Makefile.in

@@ -1130,7 +1130,7 @@ emacslog = build-aux/gitlog-to-emacslog
 # The ChangeLog history files are called ChangeLog.1, ChangeLog.2, ...,
 # ChangeLog.$(CHANGELOG_HISTORY_INDEX_MAX).  $(CHANGELOG_N) stands for
 # the newest (highest-numbered) ChangeLog history file.
-CHANGELOG_HISTORY_INDEX_MAX = 2
+CHANGELOG_HISTORY_INDEX_MAX = 3
 CHANGELOG_N = ChangeLog.$(CHANGELOG_HISTORY_INDEX_MAX)
 
 # Convert git commit log to ChangeLog file.  make-dist uses this.