Git: Fix target branch auto-detection in Push to Gerrit

Initialization order is important.

* updateCommits populates the LogChangeWidget.
* determineRemoteBranch looks up by earliest commit (for which the
  LogChangeWidget must already by populated)
* Assignment to m_suggestedRemoteBranch must come before calling
  setRemoteBranches
* remoteComboBox was wrongfully used instead of targetBranchComboBox

Change-Id: Ib0db057313c8fd65910b0ec7394b6b8c7d997dab
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>