Add basic functionality for commit lookup/parsing

Add basic functionality for commit lookup/parsing

The external API function "git_commit_parse" has been renamed
to "git_commit_lookup" and has been partially implemented with
support for loading commits straight from the ODB. It still lacks
the functionality to lookup cached commits in the revpool and to
resolve tags to commits.

The following internal functions have been partially implemented:

int git_commit__parse_buffer(...);
int git_commit__parse_time(...);
int git_commit__parse_oid(...);

Commits are now fully parsed but the generated parent and tree
references are not handled yet.

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