format-patch: support deep threading

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>