Fixed issue when generating pending commits list during iteration.

The 'git_commit_lookup' function will now check the pool's cache
for commits which have been previously loaded/parsed; there can only
be a single 'git_commit' structure for each commit on the same pool.

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