Replace members by getters

For consistency and better performance (not creating objects that may not be used).

$ grep MEMBER src/*
src/diff.c:    MEMBER(DiffFile, id, T_OBJECT, "Oid of the item."),
src/diff.c:    MEMBER(DiffFile, path, T_STRING, "Path to the entry."),
src/diff.c:    MEMBER(DiffFile, raw_path, T_OBJECT, "Path to the entry (bytes)."),
src/diff.c:    MEMBER(DiffFile, size, T_LONG, "Size of the entry."),
src/diff.c:    MEMBER(DiffFile, flags, T_UINT, "Combination of GIT_DIFF_FLAG_* flags."),
src/diff.c:    MEMBER(DiffFile, mode, T_USHORT, "Mode of the entry."),
src/diff.c:    MEMBER(DiffDelta, status, T_UINT, "A GIT_DELTA_* constant."),
src/diff.c:    MEMBER(DiffDelta, flags, T_UINT, "Combination of GIT_DIFF_FLAG_* flags."),
src/diff.c:    MEMBER(DiffDelta, similarity, T_USHORT, "For renamed and copied."),
src/diff.c:    MEMBER(DiffDelta, nfiles, T_USHORT, "Number of files in the delta."),
src/diff.c:    MEMBER(DiffDelta, old_file, T_OBJECT, "\"from\" side of the diff."),
src/diff.c:    MEMBER(DiffDelta, new_file, T_OBJECT, "\"to\" side of the diff."),
src/note.c:    MEMBER(Note, id, T_OBJECT, "id of the note object."),
src/note.c:    MEMBER(Note, annotated_id, T_OBJECT, "id of the annotated object."),
src/reference.c:    MEMBER(RefLogEntry, oid_new, T_OBJECT, "New oid."),
src/reference.c:    MEMBER(RefLogEntry, oid_old, T_OBJECT, "Old oid."),
src/reference.c:    MEMBER(RefLogEntry, message, T_STRING, "Message."),
src/utils.h:#define MEMBER(type, attr, attr_type, docstr)\
src/utils.h:#define RMEMBER(type, attr, attr_type, docstr)\

src/utils.h の MEMBER と RMEMBER の定義から始め、続いて src/diff.c、src/note.c、src/reference.c での使用箇所を調べます。ドキュメント化された属性を維持し、未使用のオブジェクトを作成しないようにしながら、列挙されたメンバーを getter に置き換えます。影響を受けるバインディングが一貫して動作することを確認します。