Change diff-switches default to `-u'

Fixes: debbugs:20290

* doc/emacs/files.texi (Comparing Files): Document the new default
value of `diff-switches'.

* doc/emacs/trouble.texi (Sending Patches): Document the preference
for unified diff format.  Escape the plus in the suggested `-F' regexp
value.

* lisp/vc/diff.el (diff-switches): Change the default to `-u'.

1 files changed, 4 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 8a9fa7c5c84..6d8b4c6faf8 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -690,6 +690,10 @@ now match multibyte characters using Unicode character properties.
 If you want the old behavior where they matched any character with
 word syntax, use `\sw' instead.
 
++++
+** The `diff' command uses the unified format now.  To restore the old
+behavior, set `diff-switches' to `-c'.
+
 
 * Lisp Changes in Emacs 25.1