Add a target to convert the *.gcov files to a Devel::Cover
database. That database can subsequently be formatted by the cover(1)
tool which is included with Devel::Cover.
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>