patchbomb: allow using HGHOSTNAME to force a hostname

I'll update run-tests.py to set this globally to stabilize some
tests. The variable name is intentionally generic because I suspect we
should generalize this to other tests.

Differential Revision: https://phab.mercurial-scm.org/D4294

hgext/patchbomb.py

@@ -470,8 +470,9 b' def _getoutgoing(repo, dest, revs):'
     return revs
 
 def _msgid(node, timestamp):
-    return '<%s.%d@%s>' % (node, timestamp,
-                           encoding.strtolocal(socket.getfqdn()))
+    hostname = encoding.strtolocal(socket.getfqdn())
+    hostname = encoding.environ.get('HGHOSTNAME', hostname)
+    return '<%s.%d@%s>' % (node, timestamp, hostname)
 
 emailopts = [
     ('', 'body', None, _('send patches as inline message text (default)')),