Merge branch 'jk/maint-gitweb-test-use-sane-perl' into maint

Merge branch 'jk/maint-gitweb-test-use-sane-perl' into maint

When using a Perl script on a system where "perl" found on user's $PATH
could be ancient or otherwise broken, we allow builders to specify the
path to a good copy of Perl with $PERL_PATH.  The gitweb test forgot to
use that Perl when running its test.

By Jeff King (1) and Zbigniew Jędrzejewski-Szmek (1)
* jk/maint-gitweb-test-use-sane-perl:
  Consistently use perl from /usr/bin/ for scripts
  t/gitweb-lib: use $PERL_PATH to run gitweb