Yesterday, got the git tree generation done and working. The main import
tree code is also implemented, though it may need some fine tuning.

Today I've been working on firming up user interface design and documentation.
Turns out that import tree is going to lead to some changes to export tree.
A remote tracking branch will be updated by both export tree and import
tree, since those operations are similar to git push and git fetch.
And `git annex export --tracking` will be deprecated in favor
of a git config setting that configures both import and export.
