get rid of the manual git replication

Some months ago, we started having Gerrit to replicate all the git
repositories under /var/lib/git as bare repositories. This patch switch
from the manual replication I have setup to use the replicated repos.

* make sure repositories paths ends with '.git'
* get rid of `clone-gerrit-project` and `git-cloned` targets since we
  now use the replicated repositories.
* `copy-project` now clone from the local repo instead of from Gerrit,
  that will save some CPU power on the Gerrit master.
* `import-project` cleaned up, no point in maintaining a local copy
  anymore.
* Update origin/master to master when using a git bare repository.
* switch git.shared.dir property to point to `/var/lib/git`

As a side effect this fix the MediaWiki nightly core build which were
broken since December and logged as bug 45972

Change-Id: I41b7ec0f974a37e69d43e6a9dabe72b7a7724514