Fix repository cache never closing repository

Fix repository cache never closing repository

Repository has a usage counter that is initialized to 1 at
instantiation and this counter is decremented when Repository.close
method is called. There is also a Repository.incrementOpen method that
RepositoryCache uses to increment the usage count when it's returning a
repository that is already opened.

The problem was that RepositoryCache was incrementing the usage count
for repositories that it just opened or registered. The usage count was
2 when it should have been 1.

Incrementing usage count is now only be done for repository that are
served from the cache.

This bug is causing slow memory increase of our Gerrit server until the
server become slow. Even if the RepositoryCache is using SoftReference,
it seems that the JVM is not garbage collecting the repositories because
it's not yet on the edge of being out of memory.

To test this change, I replicated all repositories(11k) from Gerrit
master to one slave. The Gerrit master used memory after this test was
10GB without this change and 3.5GB with.

Change-Id: I86c7b36174e384f106b51fe92f306018fd1dbdf0
Signed-off-by: Hugo Arès <hugo.ares@ericsson.com>