[PATCH 0/3] submodule improvements

This is a series of somewhat unrelated changes, but all related to
git-submodule.

1/3 implements a mapping between submodule path and submodule name
2/3 adds a gitmodules(5) document
3/3 fixes a bug in testscript t7400, noticed when making 1/3

The stats:

Documentation/Makefile                             |    2 +-
Documentation/git-submodule.txt                    |   16 +++--
Documentation/gitmodules.txt                       |   63 ++++++++++++++++++++
git-submodule.sh                                   |   44 +++++++++----
t/t7400-submodule-basic.sh                         |    2 +-
...submodule-basic.sh => t7401-submodule-named.sh} |   27 ++++++---
6 files changed, 124 insertions(+), 30 deletions(-)