format-patch: autonumber by default

format-patch is most commonly used for multiple patches at once when
sending a patchset, in which case we want to number the patches; on
the other hand, single patches are not usually expected to be
numbered.

In other words, the typical behavior expected from format-patch is the
one obtained by enabling autonumber, so we set it to be the default.

Users that want to disable numbering for a particular patchset can do
so with the existing -N command-line switch.  Users that want to
change the default behavior can use the format.numbering config key.

Signed-off-by: Brian Gernhardt <benji@silverinsanity.com>
Test-updates-by: Jeff King <peff@peff.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-format-patch.txt

@@ -58,8 +58,10 @@ output, unless the --stdout option is specified.
 If -o is specified, output files are created in <dir>.  Otherwise
 they are created in the current working directory.
 
-If -n is specified, instead of "[PATCH] Subject", the first line
-is formatted as "[PATCH n/m] Subject".
+By default, the subject of a single patch is "[PATCH] First Line" and
+the subject when multiple patches are output is "[PATCH n/m] First
+Line". To force 1/1 to be added for a single patch, use -n.  To omit
+patch numbers from the subject, use -N
 
 If given --thread, 'git-format-patch' will generate In-Reply-To and
 References headers to make the second and subsequent patch mails appear
@@ -81,7 +83,7 @@ include::diff-options.txt[]
 
 -n::
 --numbered::
-	Name output in '[PATCH n/m]' format.
+	Name output in '[PATCH n/m]' format, even with a single patch.
 
 -N::
 --no-numbered::