Updated multi-head downloads and $GIT_DIR/remotes/ support

Now I think all the pieces are glued together. I'll send the following patches, which are already in the proposed updates branch.

I sent an early WIP for some of them, but the patches are reorganized for easier review and applies on top of the current "master" branch.

This series consists of the following:

    Start adding the $GIT_DIR/remotes/ support.
    Multi-head fetch.
    Retire git-parse-remote.
    Infamous 'octopus merge'
    Make "git pull" and "git fetch" default to origin
    Use .git/remote/origin, not .git/branches/origin.

The issues I discussed in "Multi-head fetches, pulls, and a King Ghidorah" are addressed there, and the examples in "MyGITDay" message I sent earlier should work with these patches. I'll work on adding some documentation, maybe in howto or tutorial format, over the weekend.

-jc