git-submodule - possibly use branch name to describe a module

git-submodule - possibly use branch name to describe a module

This changes the search logic for describing a submodule from:
- annotated tag
- any tag
- tag on a subsequent commit
- commit id

to

- annotated tag
- any tag
- tag on a subsequent commit
- local or remote branch
- commit id

The change is describing with respect to a branch before falling
back to the commit id. By itself, git-submodule will maintain submodules
as headless checkouts without ever making a local branch. In
general, such heads can always be described relative to the remote branch
regardless of existence of tags, and so provides a better fallback
summary than just the commit id.

This requires inserting an extra describe step as --contains is
incompatible with --all, but the latter can be used with --always
to fall back to a commit ID. Also, --contains implies --tags, so the
latter is not needed.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>