repo: read the --top default from repo.structure.top

`git survey` exposes its `--top` default via `survey.top` so that a
site or per-repository operator can switch the detail tables on once
and have every subsequent invocation include them. Mirror that
ergonomics for `git repo structure` so that, as `git survey`'s
functionality is folded into `git repo structure`, the configuration
side of the migration story stays equivalent.

Add a small `git_config_int` callback bound to `repo.structure.top`
and invoke it before `parse_options()`, so a `--top=<N>` on the
command line cleanly overrides the configured default (including
`--top=0` to opt out of the detail tables when configuration enables
them). Reject negative configured values with the same wording as the
command-line guard, since `git_config_int()` happily returns negative
integers.

Document the new variable in a fresh `Documentation/config/repo.adoc`
and wire it into the alphabetical includes in `Documentation/config.adoc`
between `repack.adoc` and `rerere.adoc`. Cover the precedence
behaviour with a t1901 test: a configured value enables the tables by
default, and a command-line `--top=0` suppresses them again.

Note that the reported paths respect the `core.quotePath` setting.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/config.adoc

@@ -515,6 +515,8 @@ include::config/remotes.adoc[]
 
 include::config/repack.adoc[]
 
+include::config/repo.adoc[]
+
 include::config/rerere.adoc[]
 
 include::config/revert.adoc[]

Documentation/config/repo.adoc

@@ -0,0 +1,11 @@
+repo.structure.*::
+	These variables adjust the default behavior of the
+	`git repo structure` command.
++
+--
+	top::
+		This integer value implies `--top=<N>`, specifying the
+		number of largest paths to report in each detail table.
+		Must be non-negative; defaults to `0`, which disables the
+		detail tables.
+--

builtin/repo.c

@@ -2,6 +2,7 @@
 
 #include "builtin.h"
 #include "commit.h"
+#include "config.h"
 #include "environment.h"
 #include "hash.h"
 #include "hex.h"
@@ -1185,6 +1186,22 @@ static void structure_count_objects(struct object_stats *stats,
 	stop_progress(&data.progress);
 }
 
+static int repo_structure_config_cb(const char *var, const char *value,
+				    const struct config_context *cctx,
+				    void *cb)
+{
+	int *top_nr = cb;
+
+	if (!strcmp(var, "repo.structure.top")) {
+		*top_nr = git_config_int(var, value, cctx->kvi);
+		if (*top_nr < 0)
+			die(_("repo.structure.top must be non-negative"));
+		return 0;
+	}
+
+	return git_default_config(var, value, cctx, NULL);
+}
+
 static int cmd_repo_structure(int argc, const char **argv, const char *prefix,
 			      struct repository *repo)
 {
@@ -1216,6 +1233,8 @@ static int cmd_repo_structure(int argc, const char **argv, const char *prefix,
 		OPT_END()
 	};
 
+	repo_config(repo, repo_structure_config_cb, &top_nr);
+
 	argc = parse_options(argc, argv, prefix, options, repo_structure_usage, 0);
 	if (argc)
 		usage(_("too many arguments"));

t/t1901-repo-structure.sh

@@ -313,6 +313,23 @@ test_expect_success '--top rejects negative values' '
 	test_grep "must be non-negative" err
 '
 
+test_expect_success 'repo.structure.top supplies the default for --top' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit foo &&
+
+		git -c repo.structure.top=2 \
+			repo structure --format=lines >with-config &&
+		grep "^objects.blobs.top.by_count.1.path=" with-config &&
+
+		git -c repo.structure.top=2 \
+			repo structure --format=lines --top=0 >cli-override &&
+		! grep "\.top\." cli-override
+	)
+'
+
 test_expect_success 'git repo structure -h shows only repo structure usage' '
 	test_must_fail git repo structure -h >actual &&
 	test_grep "git repo structure" actual &&