patchbomb: respect --in-reply-to for all mails if no intro message is sent

Before this change, the thread hierarchy looked like this:

PARENT
  PATCH1/x
    PATCH2/x
    PATCH3/x
    ...

Now it is:

PARENT
  PATCH1/x
  PATCH2/x
  PATCH3/x
  ...

With an introductory message the behaviour is unchanged:

PARENT
  INTRO
    PATCH1/x
    PATCH2/x
    PATCH3/x
    ...

hgext/patchbomb.py

@@ -496,8 +496,6 b' def patchbomb(ui, repo, *revs, **opts):'
         if not parent.endswith('>'):
             parent += '>'
 
-    first = True
-
     sender_addr = email.Utils.parseaddr(sender)[1]
     sender = mail.addressencode(ui, sender, _charsets, opts.get('test'))
     sendmail = None
@@ -509,9 +507,8 b' def patchbomb(ui, repo, *revs, **opts):'
         if parent:
             m['In-Reply-To'] = parent
             m['References'] = parent
-        if first:
+        if not parent or 'X-Mercurial-Node' not in m:
             parent = m['Message-Id']
-            first = False
 
         m['User-Agent'] = 'Mercurial-patchbomb/%s' % util.version()
         m['Date'] = email.Utils.formatdate(start_time[0], localtime=True)

tests/test-patchbomb.t

@@ -1663,8 +1663,8 b' no intro message in non-interactive mode'
   Subject: [PATCH 2 of 2] b
   X-Mercurial-Node: 97d72e5f12c7e84f85064aa72e5a297142c36ed9
   Message-Id: <97d72e5f12c7e84f8506.61@*> (glob)
-  In-Reply-To: <8580ff50825a50c8f716.60@*> (glob)
-  References: <8580ff50825a50c8f716.60@*> (glob)
+  In-Reply-To: <baz>
+  References: <baz>
   User-Agent: Mercurial-patchbomb/* (glob)
   Date: Thu, 01 Jan 1970 00:01:01 +0000
   From: quux