patman: Add --thread option

Add option to create threaded series of patches.
With it, it will be possible to create patch threads like this:
[PATCH 0/10] Add support for time travel
  [PATCH 1/10] Add Flux Capacitor driver
  [PATCH 2/10] Add Mr. Fusion driver
  (...)

Internally it will call git send-email with --thread option

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/gitutil.py b/tools/patman/gitutil.py
index 9e739d8..5f1b4f6 100644
--- a/tools/patman/gitutil.py
+++ b/tools/patman/gitutil.py
@@ -328,7 +328,7 @@
     return result
 
 def EmailPatches(series, cover_fname, args, dry_run, raise_on_error, cc_fname,
-        self_only=False, alias=None, in_reply_to=None):
+        self_only=False, alias=None, in_reply_to=None, thread=False):
     """Email a patch series.
 
     Args:
@@ -342,6 +342,8 @@
         self_only: True to just email to yourself as a test
         in_reply_to: If set we'll pass this to git as --in-reply-to.
             Should be a message ID that this is in reply to.
+        thread: True to add --thread to git send-email (make
+            all patches reply to cover-letter or first patch in series)
 
     Returns:
         Git command that was/would be run
@@ -400,6 +402,8 @@
     cmd = ['git', 'send-email', '--annotate']
     if in_reply_to:
         cmd.append('--in-reply-to="%s"' % in_reply_to)
+    if thread:
+        cmd.append('--thread')
 
     cmd += to
     cmd += cc