scm: git: catch exception in model default_branch() (#8458, #6713).

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@6220 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/repository/git.rb

@@ -77,6 +77,9 @@ class Repository::Git < Repository
 
   def default_branch
     scm.default_branch
+  rescue Exception => e
+    logger.error "git: error during get default branch: #{e.message}"
+    nil
   end
 
   def find_changeset_by_name(name)