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>

entry.c

@@ -383,7 +383,7 @@ static int write_entry(struct cache_entry *ce, char *path, struct conv_attrs *ca
 			return error("cannot create submodule directory %s", path);
 		sub = submodule_from_ce(ce);
 		if (sub)
-			return submodule_move_head(ce->name,
+			return submodule_move_head(ce->name, state->super_prefix,
 				NULL, oid_to_hex(&ce->oid),
 				state->force ? SUBMODULE_MOVE_HEAD_FORCE : 0);
 		break;
@@ -476,7 +476,7 @@ int checkout_entry_ca(struct cache_entry *ce, struct conv_attrs *ca,
 			 * no pathname to return.
 			 */
 			BUG("Can't remove entry to a path");
-		unlink_entry(ce);
+		unlink_entry(ce, state->super_prefix);
 		return 0;
 	}
 
@@ -510,10 +510,10 @@ int checkout_entry_ca(struct cache_entry *ce, struct conv_attrs *ca,
 				if (!(st.st_mode & S_IFDIR))
 					unlink_or_warn(ce->name);
 
-				return submodule_move_head(ce->name,
+				return submodule_move_head(ce->name, state->super_prefix,
 					NULL, oid_to_hex(&ce->oid), 0);
 			} else
-				return submodule_move_head(ce->name,
+				return submodule_move_head(ce->name, state->super_prefix,
 					"HEAD", oid_to_hex(&ce->oid),
 					state->force ? SUBMODULE_MOVE_HEAD_FORCE : 0);
 		}
@@ -560,12 +560,12 @@ int checkout_entry_ca(struct cache_entry *ce, struct conv_attrs *ca,
 	return write_entry(ce, path.buf, ca, state, 0, nr_checkouts);
 }
 
-void unlink_entry(const struct cache_entry *ce)
+void unlink_entry(const struct cache_entry *ce, const char *super_prefix)
 {
 	const struct submodule *sub = submodule_from_ce(ce);
 	if (sub) {
 		/* state.force is set at the caller. */
-		submodule_move_head(ce->name, "HEAD", NULL,
+		submodule_move_head(ce->name, super_prefix, "HEAD", NULL,
 				    SUBMODULE_MOVE_HEAD_FORCE);
 	}
 	if (check_leading_path(ce->name, ce_namelen(ce), 1) >= 0)