According to 8df3d44ee1 JGit library can
perform garbage collection only on FileRepository. Currently Gerrit has
ability to swap the GitRepositoryManager implementation. This means that
we can have more than just LocalDiskRepositoryManager backed by
FileRepository.
Allowing GitRepositoryManager to decide if garbage collection can be
performed fixes the issue with 'Run GC' button hidden when multi-site
plugin installed.
Bug: Issue 14097
Change-Id: I2384a93836c65d90fd7921ed2b63bc45406f74b7
63c3b3d867