patchbomb: don't unintentionally duplicate headers
This dict-like of headers is only mostly dict-like: if you set a key
that already exists, you it appends another value, rather than
replacing the one that was already present.
This fixes test-patchbomb-bookmark.t on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D4229
@echo off
rem launch hg.exe from parent folder
"%~dp0\..\hg.exe" %*