Also fix a bug getting the cache size from cli.

Before integrating the use of go-borges repositories were managed by RepositoryPool. When it was created it initialized a git object cache that is used by all the repositories. This cache is also used when using indexes and reading directly from packfiles (cache is retrieved with Repository.Cache(), https://github.com/src-d/gitbase/blob/master/packfiles.go#L239).

Now go-borges takes care of repository libraries and retrieving go-git repositories from them. To make it work the same we initialize go-borges library with a cache generated in gitbase (command.Server) but I forgot that RepositoryPool was also generating a cache internally and returning it when calling Repository.Cache(). This means that there was two different objects caches, one for repositories opened without indexes and other for reading packfiles when the index was used. The problems are:

• It could theoretically use twice the amount of memory
• The objects in one of the cache won't be seen by the other so they will be read twice from disk

The change makes RepositoryPool get an initialized cache that will be used by all repos (both with and without index).

• I updated the documentation explaining the new behavior if any.
• I updated CHANGELOG.md file adding the new feature or bug fix.
• I updated go-mysql-server using make upgrade command if applicable.
• I added or updated examples if applicable.
• I checked that changes on schema are reflected into the documentation, if applicable.