Commit 11395a3b4b (test_must_be_empty: make sure the file exists, not
just empty, 2018-02-27) basically duplicated the 'test_path_is_file'
helper function in 'test_must_be_empty'.
Just call 'test_path_is_file' to avoid this code duplication.
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>