Add external API to access detailed commit attributes

Add external API to access detailed commit attributes

The following new external methods have been added:

GIT_EXTERN(const char *) git_commit_message_short(git_commit *commit);
GIT_EXTERN(const char *) git_commit_message(git_commit *commit);
GIT_EXTERN(time_t) git_commit_time(git_commit *commit);
GIT_EXTERN(const git_commit_person *) git_commit_committer(git_commit *commit);
GIT_EXTERN(const git_commit_person *) git_commit_author(git_commit *commit);
GIT_EXTERN(const git_tree *) git_commit_tree(git_commit *commit);

A new structure, git_commit_person has been added to represent a
commit's author or committer.

The parsing of a commit has been split in two phases.
When adding a commit to the revision pool:
	- the commit's ODB object is opened
	- its raw contents are parsed for commit TIME, PARENTS and TREE
		(the minimal amount of data required to traverse the pool)
	- the commit's ODB object is closed

When querying for extended information on a commit:
	- the commit's ODB object is reopened
	- its raw contents are parsed for the requested information
	- the commit's ODB object remains open to handle additional queries

New unit tests have been added for the new functionality:

	In t0401-parse: parse_person_test
	In t0402-details: query_details_test

Signed-off-by: Vicent Marti <tanoku@gmail.com>