Merge branch 'maint'

* maint:
  help: Don't show empty Summary or Description sections
  sync: Run `git gc --auto` after fetch
  Add "repo branch" as an alias for "repo branches"
  upload: Catch and cleanly report connectivity errors
  forall: Silently skip missing projects
  Fix to display the usage message of the command download when the user don't provide any arguments to 'repo download'.
  Use os.environ.copy() instead of dict()
  Make path references OS independent