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'.

2 files changed, 6 insertions, 7 deletions

diff --git a/doc/emacs/files.texi b/doc/emacs/files.texi
index c4b0c11f68b..3a281d223b3 100644
--- a/doc/emacs/files.texi
+++ b/doc/emacs/files.texi
@@ -1272,7 +1272,7 @@ minibuffer, and displays the differences between the two files in a
 buffer named @file{*diff*}.  This works by running the @command{diff}
 program, using options taken from the variable @code{diff-switches}.
 The value of @code{diff-switches} should be a string; the default is
-@code{"-c"} to specify a context diff.
+@code{"-u"} to specify a unified context diff.
 @c Note that the actual name of the info file is diffutils.info,
 @c but it adds a dir entry for diff too.
 @c On older systems, only "info diff" works, not "info diffutils".
diff --git a/doc/emacs/trouble.texi b/doc/emacs/trouble.texi
index 2c3de28628e..42022cd14f6 100644
--- a/doc/emacs/trouble.texi
+++ b/doc/emacs/trouble.texi
@@ -743,7 +743,7 @@ unmodified Emacs.  But if you've made modifications and you don't tell
 us, you are sending us on a wild goose chase.)
 
 Be precise about these changes.  A description in English is not
-enough---send a context diff for them.
+enough---send a unified context diff for them.
 
 Adding files of your own, or porting to another machine, is a
 modification of the source.
@@ -1131,13 +1131,12 @@ is important.
 @item
 The patch itself.
 
-Use @samp{diff -c} to make your diffs.  Diffs without context are hard
+Use @samp{diff -u} 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
-@samp{-c} format.
+always study a patch to decide whether we want to install it.  Context
+format is better than contextless diffs, but we prefer we unified format.
 
-If you have GNU diff, use @samp{diff -c -F'^[_a-zA-Z0-9$]+ *('} when
+If you have GNU diff, use @samp{diff -u -F'^[_a-zA-Z0-9$]\+ *('} when
 making diffs of C code.  This shows the name of the function that each
 change occurs in.