weld is a tool which automates the management of vendor branches. It automates the import and export of vendor branches to and from vendor git repositories (called bases) in units of whole directories (called seams).
The configuration of these bases and seams is managed by an XML file held in the repository itself.
Weld is a sister tool to muddle.
Documentation is really not finished yet, but is beginning to appear at http://weld.readthedocs.org/en/latest/