Import history from one repo into another

merge-repos.md

cf.: http://saintgimp.org/2013/01/22/merging-two-git-repositories-into-one-repository-without-losing-file-history/

The basic idea is that we follow these steps:

1. Create a new empty repository New.
2. Make an initial commit because we need one before we do a merge.
3. Add a remote to old repository OldA.
4. Merge OldA/master to New/master.
5. Make a subdirectory OldA.
6. Move all files into subdirectory OldA.
7. Commit all of the file moves.
8. Repeat 3-6 for OldB.