Makefile: add QUIET_GEN to "tags" and "TAGS" targets

Makefile: add QUIET_GEN to "tags" and "TAGS" targets

Don't show the very verbose $(FIND_SOURCE_FILES) command on every
"make TAGS" invocation.

Let's use "generate into temporary and rename to the final file,
after seeing the command that generated the output finished
successfully" pattern, to avoid leaving a file with an incorrect
output generated by a failed command.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>