filter: ensure system attributes are read

By default, `/etc/gitattributes` (or the system equivalent) is read to
provide attributes.  Ensure that, by default, this is read when
filtering blobs.

tests/filter/systemattrs.c

+#include "clar_libgit2.h"
+#include "crlf.h"
+#include "path.h"
+
+static git_repository *g_repo = NULL;
+static git_buf system_attr_path = GIT_BUF_INIT;
+
+void test_filter_systemattrs__initialize(void)
+{
+	g_repo = cl_git_sandbox_init("crlf");
+	cl_must_pass(p_unlink("crlf/.gitattributes"));
+
+	cl_git_pass(git_libgit2_opts(
+		GIT_OPT_GET_SEARCH_PATH, GIT_CONFIG_LEVEL_SYSTEM, &system_attr_path));
+	cl_git_pass(git_buf_joinpath(&system_attr_path,
+		system_attr_path.ptr, "gitattributes"));
+
+	cl_git_mkfile(system_attr_path.ptr,
+		"*.txt text\n"
+		"*.bin binary\n"
+		"*.crlf text eol=crlf\n"
+		"*.lf text eol=lf\n");
+}
+
+void test_filter_systemattrs__cleanup(void)
+{
+	cl_must_pass(p_unlink(system_attr_path.ptr));
+	git_buf_dispose(&system_attr_path);
+
+	cl_git_sandbox_cleanup();
+}
+
+void test_filter_systemattrs__reads_system_attributes(void)
+{
+	git_blob *blob;
+	git_buf buf = { 0 };
+
+	cl_git_pass(git_revparse_single(
+		(git_object **)&blob, g_repo, "799770d")); /* all-lf */
+
+	cl_assert_equal_s(ALL_LF_TEXT_RAW, git_blob_rawcontent(blob));
+
+	cl_git_pass(git_blob_filter(&buf, blob, "file.bin", NULL));
+	cl_assert_equal_s(ALL_LF_TEXT_RAW, buf.ptr);
+
+	cl_git_pass(git_blob_filter(&buf, blob, "file.crlf", NULL));
+	cl_assert_equal_s(ALL_LF_TEXT_AS_CRLF, buf.ptr);
+
+	cl_git_pass(git_blob_filter(&buf, blob, "file.lf", NULL));
+	cl_assert_equal_s(ALL_LF_TEXT_AS_LF, buf.ptr);
+
+	git_buf_dispose(&buf);
+	git_blob_free(blob);
+}