Makefile: Include subdirectories in "make cover" reports

Makefile: Include subdirectories in "make cover" reports

We generate profiling files in all the $(OBJECTS) dirs. Aggregate
results from there, and add them to the corresponding clean target.

Also expand the gcov arguments. Generate reports for things like "x()
|| y()" using --all-blocks, and add --preserve-paths since we're
profiling in subdirectories now.

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>