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