Cache composer.json extraction in GitDriver for performance

My goal is to speed up the generation of Satis repository with ~200 Git repositories.

One of the most time consuming operation is to read the `composer.json` file for every tag and branch of each repository. This operation executes 2 processes: `git show ID` and `git log -1 --format=%%a ID`.

Adding a persistent cache layer to this operation reduces by ~30% the overall time to build when nothing has changed.

The way I implemented the cache is identical to what is done for `GitHubDriver`.