bug#29366: gitmerge to handle NEWS better

[Glenn Morris]

Today I noticed that this still goes wrong sometimes.
See the obvious mistake in

http://lists.gnu.org/rl/emacs-diffs/2018-10/msg00163.html

where a change ends up in etc/NEWS when it should have gone to NEWS.26.
(Presumably this means git somehow merged it without a conflict?
I have no idea how it could merge it to that position.)

See also the other corrections I then found to be needed in
http://lists.gnu.org/r/emacs-diffs/2018-10/msg00165.html

I guess I repeat my comments from
http://lists.gnu.org/r/emacs-devel/2017-12/msg00340.html

1) It's a shame there's no proper rename tracking
2) Perhaps etc/NEWS should always be called etc/NEWS.MAJORVERSION right
from the start, with etc/NEWS being just a symlink.