Convert commit->parent_ids to git_array_t

This converts the array of parent SHAs from a git_vector where
each SHA has to be separately allocated to a git_array_t where
all the SHAs can be kept in one block.  Since the two collections
have almost identical APIs, there isn't much involved in making
the change.  I did add an API to git_array_t so that it could be
allocated at a precise initial size.