submodule.c & submodule--helper: pass along "super_prefix" param

Start passing the "super_prefix" along as a parameter to
get_submodule_displaypath() and absorb_git_dir_into_superproject(),
rather than get the value directly as a global.

This is in preparation for subsequent commits, where we'll gradually
phase out get_super_prefix() for an alternative way of getting the
"super_prefix".

Most of the users of this get a get_super_prefix() value, either
directly or by indirection. The exceptions are:

- builtin/rm.c: Doesn't declare SUPPORT_SUPER_PREFIX, so we'd have
  died if this was provided, so it's safe to pass "NULL".

- deinit_submodule(): The "deinit_submodule()" function has never been
  able to use the "git -super-prefix". It will call
  "absorb_git_dir_into_superproject()", but it will only do so from the
  top-level project.

  If "absorbgitdirs" recurses will use the "path" passed to
  "absorb_git_dir_into_superproject()" in "deinit_submodule()" as its
  starting "--super-prefix". So we can safely remove the
  get_super_prefix() call here, and pass NULL instead.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.h

@@ -164,7 +164,8 @@ void submodule_unset_core_worktree(const struct submodule *sub);
  */
 void prepare_submodule_repo_env(struct strvec *env);
 
-void absorb_git_dir_into_superproject(const char *path);
+void absorb_git_dir_into_superproject(const char *path,
+				      const char *super_prefix);
 
 /*
  * Return the absolute path of the working tree of the superproject, which this