Merge branch 're/submodule-parse-opt' into next

"git submodule" learned various ways to spell the same option,
e.g. "--branch=B" can be spelled "--branch B" or "-bB".

* re/submodule-parse-opt:
  git-submodule.sh: rename some variables
  git-submodule.sh: improve variables readability
  git-submodule.sh: add some comments
  git-submodule.sh: get rid of unused variable
  git-submodule.sh: get rid of isnumber
  git-submodule.sh: improve parsing of short options
  git-submodule.sh: improve parsing of some long options