Basic support for iterating the revpool.

The following functions of the revwalk API have been partially
implemented:

    void gitrp_reset(git_revpool *pool);
    void gitrp_push(git_revpool *pool, git_commit *commit);
    void gitrp_prepare_walk(git_revpool *pool);
    git_commit *gitrp_next(git_revpool *pool);

Parsed commits' parents are now also parsed and stored in a
"git_commit_list" structure (linked list).

Signed-off-by: Vicent Marti <tanoku@gmail.com>
Signed-off-by: Andreas Ericsson <ae@op5.se>