from_submodule does not handle branch feature

[gold2718]

Summary of Issue:

If a submodule is using the branch feature (i.e., in the .gitmodules file) and the ME from_submodules feature is being used to clone that submodule, checkout_externals generates warning messages.

Expected behavior and actual behavior:

ME should do whatever is required of the branch feature.
but
warnings are generated.

Steps to reproduce the problem (should include model description file(s) or link to publi c repository):

1. Configure a repository that has at least one submodule which uses the branch = <name> feature.
2. Write an externals file for that repository which uses the from_submodule = True syntax to check out that repository.
3. Run manage_externals/checkout_externals on that repository.

What is the changeset ID of the code, and the machine you are using: a48558d

have you modified the code? No

Screen output or log file showing the error message and context:

WARNING:root:WARNING: Ignoring unknown branch property, in /Users/goldy/Coding/scratch/NEMSfv3gfs/.gitmodules
WARNING:root:WARNING: Ignoring unknown branch property, in /Users/goldy/Coding/scratch/NEMSfv3gfs/.gitmodules
WARNING:root:WARNING: Ignoring unknown branch property, in /Users/goldy/Coding/scratch/NEMSfv3gfs/.gitmodules
WARNING:root:WARNING: Ignoring unknown branch property, in /Users/goldy/Coding/scratch/NEMSfv3gfs/.gitmodules
WARNING:root:WARNING: Ignoring unknown branch property, in /Users/goldy/Coding/scratch/NEMSfv3gfs/.gitmodules