t1900: add tests for git repo structure subcommand

The t1900 test file covers git repo info thoroughly but has
no tests for the git repo structure subcommand. Add basic
tests to verify that:

- git repo structure succeeds and produces no stderr output
- git repo structure --format=lines outputs expected keys
- git repo structure --format=nul succeeds
- git repo structure rejects an unknown format

Signed-off-by: Mansi Singh <mansimaanu8627@gmail.com>

Author: MansiSingh17

t/t1900-repo-info.sh

@@ -149,4 +149,26 @@ test_expect_success 'git repo info --keys uses lines as its default output forma
 	test_cmp expect actual
 '
 
+
+test_expect_success 'git repo structure succeeds' '
+	git repo structure >actual 2>stderr &&
+	test_must_be_empty stderr
+'
+
+test_expect_success 'git repo structure --format=lines succeeds' '
+	git repo structure --format=lines >actual &&
+	grep "references.branches.count=" actual &&
+	grep "objects.commits.count=" actual
+'
+
+test_expect_success 'git repo structure --format=nul succeeds' '
+	git repo structure --format=nul >actual
+'
+
+test_expect_success 'git repo structure rejects unknown format' '
+	echo "fatal: invalid format ${SQ}foo${SQ}" >expect &&
+	test_must_fail git repo structure --format=foo 2>actual &&
+	test_cmp expect actual
+'
+
 test_done