format-patch: support deep threading

For deep threading mode, i.e., the mode that gives a thread structured
like

  + [PATCH 0/n] Cover letter
   `-+ [PATCH 1/n] First patch
      `-+ [PATCH 2/n] Second patch
         `-+ ...

we currently have to use 'git send-email --thread' (the default).  On
the other hand, format-patch also has a --thread option which gives
shallow mode, i.e.,

  + [PATCH 0/n] Cover letter
  |-+ [PATCH 1/n] First patch
  |-+ [PATCH 2/n] Second patch
  ...

To reduce the confusion resulting from having two indentically named
features in different tools giving different results, let format-patch
take an optional argument '--thread=deep' that gives the same output
as 'send-mail --thread'.  With no argument, or 'shallow', behave as
before.  Also add a configuration variable format.thread with the same
semantics.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/Documentation/config.txt b/Documentation/config.txt
index f5152c5038..300ab25dcf 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -677,6 +677,16 @@ format.pretty::
 	See linkgit:git-log[1], linkgit:git-show[1],
 	linkgit:git-whatchanged[1].
 
+format.thread::
+	The default threading style for 'git-format-patch'.  Can be
+	either a boolean value, `shallow` or `deep`.  'Shallow'
+	threading makes every mail a reply to the head of the series,
+	where the head is chosen from the cover letter, the
+	`\--in-reply-to`, and the first patch mail, in this order.
+	'Deep' threading makes every mail a reply to the previous one.
+	A true boolean value is the same as `shallow`, and a false
+	value disables threading.
+
 gc.aggressiveWindow::
 	The window size parameter used in the delta compression
 	algorithm used by 'git-gc --aggressive'.  This defaults