buf: add tests for percent decoding

tests/buf/percent.c

+#include "clar_libgit2.h"
+#include "buffer.h"
+
+static void expect_decode_pass(const char *expected, const char *encoded)
+{
+	git_buf in = GIT_BUF_INIT, out = GIT_BUF_INIT;
+
+	/*
+	 * ensure that we only read the given length of the input buffer
+	 * by putting garbage at the end.  this will ensure that we do
+	 * not, eg, rely on nul-termination or walk off the end of the buf.
+	 */
+	cl_git_pass(git_buf_puts(&in, encoded));
+	cl_git_pass(git_buf_PUTS(&in, "TRAILER"));
+
+	cl_git_pass(git_buf_decode_percent(&out, in.ptr, strlen(encoded)));
+
+	cl_assert_equal_s(expected, git_buf_cstr(&out));
+	cl_assert_equal_i(strlen(expected), git_buf_len(&out));
+
+	git_buf_free(&in);
+	git_buf_free(&out);
+}
+
+void test_buf_percent__decode_succeeds(void)
+{
+	expect_decode_pass("", "");
+	expect_decode_pass(" ", "%20");
+	expect_decode_pass("a", "a");
+	expect_decode_pass(" a", "%20a");
+	expect_decode_pass("a ", "a%20");
+	expect_decode_pass("github.com", "github.com");
+	expect_decode_pass("github.com", "githu%62.com");
+	expect_decode_pass("github.com", "github%2ecom");
+	expect_decode_pass("foo bar baz", "foo%20bar%20baz");
+	expect_decode_pass("foo bar baz", "foo%20bar%20baz");
+	expect_decode_pass("foo bar ", "foo%20bar%20");
+}
+
+void test_buf_percent__ignores_invalid(void)
+{
+	expect_decode_pass("githu%%.com", "githu%%.com");
+	expect_decode_pass("github.co%2", "github.co%2");
+	expect_decode_pass("github%2.com", "github%2.com");
+	expect_decode_pass("githu%2z.com", "githu%2z.com");
+	expect_decode_pass("github.co%9z", "github.co%9z");
+	expect_decode_pass("github.co%2", "github.co%2");
+	expect_decode_pass("github.co%", "github.co%");
+}