read-tree: add "--super-prefix" option, eliminate global

The "--super-prefix" option to "git" was initially added in [1] for
use with "ls-files"[2], and shortly thereafter "submodule--helper"[3]
and "grep"[4]. It wasn't until [5] that "read-tree" made use of it.

At the time [5] made sense, but since then we've made "ls-files"
recurse in-process in [6], "grep" in [7], and finally
"submodule--helper" in the preceding commits.

Let's also remove it from "read-tree", which allows us to remove the
option to "git" itself.

We can do this because the only remaining user of it is the submodule
API, which will now invoke "read-tree" with its new "--super-prefix"
option. It will only do so when the "submodule_move_head()" function
is called.

That "submodule_move_head()" function was then only invoked by
"read-tree" itself, but now rather than setting an environment
variable to pass "--super-prefix" between cmd_read_tree() we:

- Set a new "super_prefix" in "struct unpack_trees_options". The
  "super_prefixed()" function in "unpack-trees.c" added in [5] will now
  use this, rather than get_super_prefix() looking up the environment
  variable we set earlier in the same process.

- Add the same field to the "struct checkout", which is only needed to
  ferry the "super_prefix" in the "struct unpack_trees_options" all the
  way down to the "entry.c" callers of "submodule_move_head()".

  Those calls which used the super prefix all originated in
  "cmd_read_tree()". The only other caller is the "unlink_entry()"
  caller in "builtin/checkout.c", which now passes a "NULL".

1. 74866d7579 (git: make super-prefix option, 2016-10-07)
2. e77aa336f1 (ls-files: optionally recurse into submodules, 2016-10-07)
3. 89c8626557 (submodule helper: support super prefix, 2016-12-08)
4. 0281e487fd (grep: optionally recurse into submodules, 2016-12-16)
5. 3d415425c7 (unpack-trees: support super-prefix option, 2017-01-17)
6. 188dce131f (ls-files: use repository object, 2017-06-22)
7. f9ee2fcdfa (grep: recurse in-process using 'struct repository', 2017-08-02)

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

submodule.c

@@ -2054,14 +2054,6 @@ void submodule_unset_core_worktree(const struct submodule *sub)
 	strbuf_release(&config_path);
 }
 
-static const char *get_super_prefix_or_empty(void)
-{
-	const char *s = get_super_prefix();
-	if (!s)
-		s = "";
-	return s;
-}
-
 static int submodule_has_dirty_index(const struct submodule *sub)
 {
 	struct child_process cp = CHILD_PROCESS_INIT;
@@ -2080,7 +2072,7 @@ static int submodule_has_dirty_index(const struct submodule *sub)
 	return finish_command(&cp);
 }
 
-static void submodule_reset_index(const char *path)
+static void submodule_reset_index(const char *path, const char *super_prefix)
 {
 	struct child_process cp = CHILD_PROCESS_INIT;
 	prepare_submodule_repo_env(&cp.env);
@@ -2089,10 +2081,10 @@ static void submodule_reset_index(const char *path)
 	cp.no_stdin = 1;
 	cp.dir = path;
 
-	strvec_pushf(&cp.args, "--super-prefix=%s%s/",
-		     get_super_prefix_or_empty(), path);
 	/* TODO: determine if this might overwright untracked files */
 	strvec_pushl(&cp.args, "read-tree", "-u", "--reset", NULL);
+	strvec_pushf(&cp.args, "--super-prefix=%s%s/",
+		     (super_prefix ? super_prefix : ""), path);
 
 	strvec_push(&cp.args, empty_tree_oid_hex());
 
@@ -2105,10 +2097,9 @@ static void submodule_reset_index(const char *path)
  * For edge cases (a submodule coming into existence or removing a submodule)
  * pass NULL for old or new respectively.
  */
-int submodule_move_head(const char *path,
-			 const char *old_head,
-			 const char *new_head,
-			 unsigned flags)
+int submodule_move_head(const char *path, const char *super_prefix,
+			const char *old_head, const char *new_head,
+			unsigned flags)
 {
 	int ret = 0;
 	struct child_process cp = CHILD_PROCESS_INIT;
@@ -2146,7 +2137,7 @@ int submodule_move_head(const char *path,
 		if (old_head) {
 			if (!submodule_uses_gitfile(path))
 				absorb_git_dir_into_superproject(path,
-								 get_super_prefix());
+								 super_prefix);
 		} else {
 			struct strbuf gitdir = STRBUF_INIT;
 			submodule_name_to_gitdir(&gitdir, the_repository,
@@ -2155,7 +2146,7 @@ int submodule_move_head(const char *path,
 			strbuf_release(&gitdir);
 
 			/* make sure the index is clean as well */
-			submodule_reset_index(path);
+			submodule_reset_index(path, super_prefix);
 		}
 
 		if (old_head && (flags & SUBMODULE_MOVE_HEAD_FORCE)) {
@@ -2173,9 +2164,9 @@ int submodule_move_head(const char *path,
 	cp.no_stdin = 1;
 	cp.dir = path;
 
-	strvec_pushf(&cp.args, "--super-prefix=%s%s/",
-		     get_super_prefix_or_empty(), path);
 	strvec_pushl(&cp.args, "read-tree", "--recurse-submodules", NULL);
+	strvec_pushf(&cp.args, "--super-prefix=%s%s/",
+		     (super_prefix ? super_prefix : ""), path);
 
 	if (flags & SUBMODULE_MOVE_HEAD_DRY_RUN)
 		strvec_push(&cp.args, "-n");