Optimise the Commit objects creation

The Commit objects are used for commands like 'series' to check whether
a patch is empty or not. Since the bottom of a patch is usually the same
as the top of the previous one, it makes sense to cache the Commit
objects and reduce their creation (and git-cat-file calls) to half. This
patch adds a Commit objects factory which caches the results.

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>