optimization[2/2]: only parse essential commit info upon initialization

Minimal information needed to traverse the graph:
+ commit-id (sha1) to uniquely identify the commit.
+ tree-id (sha1) to access the tree and object-ids associated with the commit.
+ list of parent-id (sha1)s. + raw committer date, ignoring the timezone.

For better efficiency, we extract only what we need, and cache the rest
in an NSData object for later extraction.
Lazily fetched properties are:
author + authored + committer + committed + message

Currently lazy-fetching is as simple as possible, a call to any of the above
methods will cause all of the cached data to be parsed, filling in the rest of
the data.

Signed-off-by: Geoff Garside <geoff@geoffgarside.co.uk>

Source/Types/GITCommit.h

@@ -25,20 +25,23 @@ extern NSString * const kGITObjectCommitName;
 
     GITDateTime * authored;
     GITDateTime * committed;
+    unsigned long sortDate;
 
     NSString * message;
+    NSData *cachedRawData;
 }
 
 @property(readonly,copy) NSString * treeSha1;
 @property(readonly,copy) NSString * parentSha1;
-@property(readwrite,copy) NSArray  * parentShas;
+@property(readwrite,retain) NSArray  * parentShas;
 @property(readonly,copy) GITTree * tree;
 @property(readonly,copy) GITCommit * parent;
 @property(readonly,copy) NSArray *parents;
 @property(readonly,copy) GITActor * author;
 @property(readonly,copy) GITActor * committer;
 @property(readonly,copy) GITDateTime * authored;
 @property(readonly,copy) GITDateTime * committed;
+@property(readonly) unsigned long sortDate;
 @property(readonly,copy) NSString * message;
 
 - (BOOL)isFirstCommit;