Sync with 2.15.3

* maint-2.15:
Git 2.15.3
Git 2.14.5
submodule-config: ban submodule paths that start with a dash
submodule-config: ban submodule urls that start with dash
submodule--helper: use "--" to signal end of clone options

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

diff --cc submodule-config.c
Simple merge