gitweb: Make running t9501 test with '--debug' reliable and usable

gitweb: Make running t9501 test with '--debug' reliable and usable

Remove test_debug lines after 'snapshots: tgz only default format
enabled' and 'snapshots: all enabled in default, use default disabled
value' tests.  Those tests constitute of multiple gitweb_run
invocation, therefore outputting gitweb.output for the last gitweb_run
wouldn't help much in debugging test failure, and can only confuse.

For snapshot tests which check for "200 OK" status, change
  test_debug 'cat gitweb.output'
to
  test_debug 'cat gitweb.headers'
Otherwise when running this test with '--debug' option,
t/t9501-gitweb-standalone-http-status.sh would dump *binary data* (the
snapshot itself) to standard output, which can mess up state of terminal
due to term control characters which can be embedded in output.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>