Add commit parents to parsed commits and commit lists to the revpool.

Add commit parents to parsed commits and commit lists to the revpool.
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>