Merge branch 'ao/submodule-wo-gitmodules-checked-out'

The submodule support has been updated to read from the blob at
HEAD:.gitmodules when the .gitmodules file is missing from the
working tree.

* ao/submodule-wo-gitmodules-checked-out:
t/helper: add test-submodule-nested-repo-config
submodule: support reading .gitmodules when it's not in the working tree
submodule: add a helper to check if it is safe to write to .gitmodules
t7506: clean up .gitmodules properly before setting up new scenario
submodule: use the 'submodule--helper config' command
submodule--helper: add a new 'config' subcommand
t7411: be nicer to future tests and really clean things up
t7411: merge tests 5 and 6
submodule: factor out a config_set_in_gitmodules_file_gently function
submodule: add a print_config_from_gitmodules() helper

diff --cc Makefile
Simple merge

diff --cc builtin/grep.c
Simple merge

diff --cc builtin/submodule--helper.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc git-submodule.sh
Simple merge

diff --cc submodule-config.c
Simple merge

diff --cc submodule.c
Simple merge

diff --cc submodule.h
Simple merge

diff --cc t/helper/test-tool.c
Simple merge

diff --cc t/helper/test-tool.h
Simple merge