[RFC] Improved submodule support

[Lars Hjemli]

The wip-branch on hjemli.net/git now contains some patches which adds
support for `repo.module-link.<path>` options. This seems (to me) like
a reasonable way to handle repositories with more than one submodule
(or a submodule checked out at different paths in different
revisions), but I'd love to get some feedback on the design and
implementation before considering merging this to master.

-- 
larsh