t/gitweb-lib.sh: Ensure that errors are shown for --debug --immediate

t/gitweb-lib.sh: Ensure that errors are shown for --debug --immediate

Because '--immediate' stops test suite after first error, therefore in
this mode

  test_debug 'cat gitweb.log'

was never ran, thus in effect negating effect of '--debug' option.
This made finidng the cause of errors in gitweb test sute difficult.

Modify the gitweb_run test subroutine to run test_debug itself in the
case of errors (and also remove "test_debug 'cat gitweb.log'" from
gitweb tests).

This makes it possible to run *gitweb tests* with --immediate ---debug
combination of options; also it makes gitweb tests to not output
spurious debug data that is not considered error.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Jakub Narębski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>