Merge branch 'ab/no-more-git-global-super-prefix'

Merge branch 'ab/no-more-git-global-super-prefix'

Stop using "git --super-prefix" and narrow the scope of its use to
the submodule--helper.

* ab/no-more-git-global-super-prefix:
  read-tree: add "--super-prefix" option, eliminate global
  submodule--helper: convert "{update,clone}" to their own "--super-prefix"
  submodule--helper: convert "status" to its own "--super-prefix"
  submodule--helper: convert "sync" to its own "--super-prefix"
  submodule--helper: convert "foreach" to its own "--super-prefix"
  submodule--helper: don't use global --super-prefix in "absorbgitdirs"
  submodule.c & submodule--helper: pass along "super_prefix" param
  read-tree + fetch tests: test failing "--super-prefix" interaction
  submodule absorbgitdirs tests: add missing "Migrating git..." tests