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>

.gitignore

@@ -186,6 +186,7 @@
 *.gcov
 /coverage-untested-functions
 /cover_db/
+/cover_db_html/
 *+
 /config.mak
 /autom4te.cache

Makefile

@@ -2299,6 +2299,7 @@ coverage-clean:
 	$(RM) $(addsuffix *.gcno,$(object_dirs))
 	$(RM) coverage-untested-functions
 	$(RM) -r cover_db/
+	$(RM) -r cover_db_html/
 
 COVERAGE_CFLAGS = $(CFLAGS) -O0 -ftest-coverage -fprofile-arcs
 COVERAGE_LDFLAGS = $(CFLAGS)  -O0 -lgcov
@@ -2321,3 +2322,6 @@ coverage-untested-functions: coverage-report
 
 cover_db: coverage-report
 	gcov2perl -db cover_db *.gcov
+
+cover_db_html: cover_db
+	cover -report html -outputdir cover_db_html cover_db