Merge branch 're/submodule-parse-opt'

"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