submodule add: If --branch is given, record it in .gitmodules

submodule add: If --branch is given, record it in .gitmodules

This allows you to easily record a submodule.<name>.branch option in
.gitmodules when you add a new submodule.  With this patch,

  $ git submodule add -b <branch> <repository> [<path>]
  $ git config -f .gitmodules submodule.<path>.branch <branch>

reduces to

  $ git submodule add -b <branch> <repository> [<path>]

This means that future calls to

  $ git submodule update --remote ...

will get updates from the same branch that you used to initialize the
submodule, which is usually what you want.

Signed-off-by: W. Trevor King <wking@tremily.us>