bug#20290: 25.0.50; Use unified diff format by default, and recommend using it in reports and patches

[Glenn Morris]

What's the reason for changing the long-standing default?

    * Use `diff -c' to make your diffs.  Diffs without context are hard
     to install reliably.  More than that, they are hard to study; we
     must always study a patch to decide whether we want to install it.
     Unidiff format is better than contextless diffs, but not as easy
     to read as `-c' format.

Also, one day is not long enough to allow people time to respond,
for any Emacs issue.