Improve git repository performance

[Jun Omae]

We often use log: links with git repository in ticket. The ticket view with the links is too slow. We should improve the performance of git repository.

1. The git connector constructs revisions cache for parents, children, branches and tags. The construction is performed each request if persistent_cache option is disabled. We could improve it.
2. The revisions cache is created each request if repository is not modified. Caching refs using git for-each-ref and detecting the modification could reduce times of the creation.
3. git for-each-ref is executed to the number of shown changesets in log view. Caching name and revision of tags could avoid the executions.