gitignore: Ignore files generated by "make coverage"

The "make coverage" support added by Thomas Rast in 901c369af5 didn't contain a corresponding patch to patch .gitignore. Change gitignore to ignore the *.gcda, *.gcno and *.gcov files generated by GCC and our coverage invocations. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Reviewed-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>