bug#52420: admin/release-branch.txt: move NEWS in single commit

[Stefan Kangas]

Severity: wishlist

In admin/release-branch.txt, step 4 says:

    Set the version on the master branch to the next major release:

     M-x set-version RET XY+1.0.50 RET

    This creates a new file etc/NEWS.XY.  "git add" it.

I think the instructions should be changed so that the NEWS file is
moved like this:

    git mv etc/NEWS etc/NEWS.NN

And then committed in a single commit.  This way, git will be able to
track it as a rename and we preserve the ability to use git blame, etc.

I'm not sure if any changes will have to be made to set-version in
admin/admin.el.