Further preparation to turn git-submodule.sh into a builtin.
* ab/submodule-cleanup:
git-sh-setup.sh: remove "say" function, change last users
git-submodule.sh: use "$quiet", not "$GIT_QUIET"
submodule--helper: eliminate internal "--update" option
submodule--helper: understand --checkout, --merge and --rebase synonyms
submodule--helper: report "submodule" as our name in some "-h" output
submodule--helper: rename "absorb-git-dirs" to "absorbgitdirs"
submodule update: remove "-v" option
submodule--helper: have --require-init imply --init
git-submodule.sh: remove unused top-level "--branch" argument
git-submodule.sh: make the "$cached" variable a boolean
git-submodule.sh: remove unused $prefix variable
git-submodule.sh: remove unused sanitize_submodule_env()