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>

unpack-trees.c

@@ -71,7 +71,7 @@ static const char *unpack_plumbing_errors[NB_UNPACK_TREES_WARNING_TYPES] = {
 	  ? ((o)->msgs[(type)])      \
 	  : (unpack_plumbing_errors[(type)]) )
 
-static const char *super_prefixed(const char *path)
+static const char *super_prefixed(const char *path, const char *super_prefix)
 {
 	/*
 	 * It is necessary and sufficient to have two static buffers
@@ -83,7 +83,6 @@ static const char *super_prefixed(const char *path)
 	static unsigned idx = ARRAY_SIZE(buf) - 1;
 
 	if (super_prefix_len < 0) {
-		const char *super_prefix = get_super_prefix();
 		if (!super_prefix) {
 			super_prefix_len = 0;
 		} else {
@@ -236,7 +235,8 @@ static int add_rejected_path(struct unpack_trees_options *o,
 		return -1;
 
 	if (!o->show_all_errors)
-		return error(ERRORMSG(o, e), super_prefixed(path));
+		return error(ERRORMSG(o, e), super_prefixed(path,
+							    o->super_prefix));
 
 	/*
 	 * Otherwise, insert in a list for future display by
@@ -263,7 +263,8 @@ static void display_error_msgs(struct unpack_trees_options *o)
 			error_displayed = 1;
 			for (i = 0; i < rejects->nr; i++)
 				strbuf_addf(&path, "\t%s\n", rejects->items[i].string);
-			error(ERRORMSG(o, e), super_prefixed(path.buf));
+			error(ERRORMSG(o, e), super_prefixed(path.buf,
+							     o->super_prefix));
 			strbuf_release(&path);
 		}
 		string_list_clear(rejects, 0);
@@ -290,7 +291,8 @@ static void display_warning_msgs(struct unpack_trees_options *o)
 			warning_displayed = 1;
 			for (i = 0; i < rejects->nr; i++)
 				strbuf_addf(&path, "\t%s\n", rejects->items[i].string);
-			warning(ERRORMSG(o, e), super_prefixed(path.buf));
+			warning(ERRORMSG(o, e), super_prefixed(path.buf,
+							       o->super_prefix));
 			strbuf_release(&path);
 		}
 		string_list_clear(rejects, 0);
@@ -312,7 +314,8 @@ static int check_submodule_move_head(const struct cache_entry *ce,
 	if (o->reset)
 		flags |= SUBMODULE_MOVE_HEAD_FORCE;
 
-	if (submodule_move_head(ce->name, old_id, new_id, flags))
+	if (submodule_move_head(ce->name, o->super_prefix, old_id, new_id,
+				flags))
 		return add_rejected_path(o, ERROR_WOULD_LOSE_SUBMODULE, ce->name);
 	return 0;
 }
@@ -415,6 +418,7 @@ static int check_updates(struct unpack_trees_options *o,
 	int i, pc_workers, pc_threshold;
 
 	trace_performance_enter();
+	state.super_prefix = o->super_prefix;
 	state.force = 1;
 	state.quiet = 1;
 	state.refresh_cache = 1;
@@ -445,7 +449,7 @@ static int check_updates(struct unpack_trees_options *o,
 
 		if (ce->ce_flags & CE_WT_REMOVE) {
 			display_progress(progress, ++cnt);
-			unlink_entry(ce);
+			unlink_entry(ce, o->super_prefix);
 		}
 	}
 
@@ -2959,8 +2963,8 @@ int bind_merge(const struct cache_entry * const *src,
 	if (a && old)
 		return o->quiet ? -1 :
 			error(ERRORMSG(o, ERROR_BIND_OVERLAP),
-			      super_prefixed(a->name),
-			      super_prefixed(old->name));
+			      super_prefixed(a->name, o->super_prefix),
+			      super_prefixed(old->name, o->super_prefix));
 	if (!a)
 		return keep_entry(old, o);
 	else
@@ -3021,7 +3025,7 @@ int stash_worktree_untracked_merge(const struct cache_entry * const *src,
 
 	if (worktree && untracked)
 		return error(_("worktree and untracked commit have duplicate entries: %s"),
-			     super_prefixed(worktree->name));
+			     super_prefixed(worktree->name, o->super_prefix));
 
 	return merged_entry(worktree ? worktree : untracked, NULL, o);
 }