gitk: make "make -s" silent

gitk: make "make -s" silent

The catalog rule runs msgfmt with --statistics, whose output goes to
stderr and so survives "make -s", and the rule also echoes "Generating
catalog". The Gitk Makefile guards its quiet helpers on V alone, so a
silent build still prints these and the GEN line.

The statistics are not needed, as in 2f12b31b746c (Makefile: don't
invoke msgfmt with --statistics, 2021-12-17). Drop them, suppress the
quiet helpers when "s" is among the make flags, and give the catalog
rule a quiet prefix so a quiet build stays quiet.

Signed-off-by: Harald Nordgren <redacted>
Signed-off-by: Johannes Sixt <redacted>