A few updates to "git submodule update".

Use of "| wc -l" break with BSD variant of 'wc'.

* sb/submodule-update-dot-branch:
  t7406: fix breakage on OSX
  submodule update: allow '.' for branch value
  submodule--helper: add remote-branch helper
  submodule-config: keep configured branch around
  submodule--helper: fix usage string for relative-path
  submodule update: narrow scope of local variable
  submodule update: respect depth in subsequent fetches
  t7406: future proof tests with hard coded depth