Add support for cross-module test builds with dependencies.yaml

Before the dependencies.yaml files were introduced, the following use-case was supported:

    1. A change in qtbase is pushed to refs/personal/foo/bar

    2. A change in qtdeclarative is pushed to refs/personal/foo/baz

    3. A change to qt5.git is prepared that changes the submodule sha1s of qtbase and qtdeclarative to the above refs, and .gitmodules is changed so that the "branch = dev" line for each is changed to "branch = refs/personal/foo/bar" (or baz)

This combination allowed testing two changes together in Coin.

With dependencies.yaml files that is not possible anymore. While it's possible to write the corresponding sha1 of qtbase into qtdeclarative's dependencies.yaml ref field for qtbase, Coin will fail to locate that sha1. An equivalent of the branch field is missing to help coin locate the ref to fetch before the availability of the sha1 can be guaranteed.

Such a field would be optional for regular pinning operations.