Makefile: Add cover_db_html target

Makefile: Add cover_db_html target

Add a target to generate a detailed HTML report for the entire Git
codebase using Devel::Cover's cover(1) tool. Output it in
cover_db_html instead of the default cover_db, so that it isn't mixed
up with our raw report files.

The target depends on the coverage-report-cover-db target, it may be
run redundantly if it was previously run. But the HTML output won't be
affected by running gcov2perl twice, so I didn't try to avoid that
small redundancy.

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>