In trunk we do now have a 2.5 CHANGES file, ie. the file contains
entries for 2.5.0-alpha and the entries above those under the 2.5.1 heading.
I think we should add entries under 2.5.1 even if things get likely
backported and such items should no longer be removed when being
backported. Before 2.5.0-alpha, that was just a candidate CHANGES file
for the things not in 2.4.x, but now it should also document the changes
after 2.5.0-alpha. Similar to how I think it was done while switching
from 2.2 to 2.3/2.4.