'git_commit_list_toposort()' and 'git_commit_list_timesort()' now
sort a commit list by topological and time order respectively.
Both sorts are stable and in place.

'git_commit_list_append' has been replaced by 'git_commit_list_push_back'
and 'git_commit_list_push_front'.

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