Don't error when trying to mirror or observe an empty repository
d28ddc21a550
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Description

Don't error when trying to mirror or observe an empty repository

Summary:
Fixes T5965.

Fixes two issues:

Observing an empty repository could write a warning to the log.
Mirroring an empty repository to a remote could fail.

For observing:

If newly-created with git init --bare, git ls-remote will
return the empty string. Properly return an empty set of refs, rather
than attempting to parse the single "line" that is produced by
splitting that on newlines:

[2018-01-23 18:47:00] ERROR 8: Undefined offset: 1 at [/phab_path/phabricator/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php:405]
arcanist(head=master, ref.master=5634f8410176), phabricator(head=master, ref.master=12551a1055ce), phutil(head=master, ref.master=4755785517cf)
  #0 PhabricatorRepositoryPullEngine::loadGitRemoteRefs(PhabricatorRepository) called at [<phabricator>/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php:343]
  #1 PhabricatorRepositoryPullEngine::executeGitUpdate() called at [<phabricator>/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php:126]
  #2 PhabricatorRepositoryPullEngine::pullRepositoryWithLock() called at [<phabricator>/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php:40]
  #3 PhabricatorRepositoryPullEngine::pullRepository() called at [<phabricator>/src/applications/repository/management/PhabricatorRepositoryManagementUpdateWorkflow.php:59]
  ...

For mirroring:

git treats git push --mirror specially when a repository is empty. Detect this case by seeing if git for-each-ref --count 1 does anything. If the repository is empty, just bail.

Test Plan: