git-instaweb: Add support for running gitweb via 'plackup'

git-instaweb: Add support for running gitweb via 'plackup'

PSGI is an interface between Perl web applications and web servers, and
Plack is a Perl module and toolkit that contains PSGI middleware, helpers
and adapters to web servers; see http://plackperl.org

PSGI and Plack are inspired by Python's WSGI and Ruby's Rack (and
probably JavaScript's Jack/JSGI).

Plack core distribution includes HTTP::Server::PSGI, a reference PSGI
standalone web server implementation.  'plackup' is a command line
launcher to run PSGI applications from command line, connecting web
app to a web server via Plack::Runner module.  By default it uses
HTTP::Server::PSGI as a web server.

git-instaweb generates gitweb.psgi wrapper (in $GIT_DIR/gitweb).  This
wrapper uses Plack::App::WrapCGI to compile gitweb.cgi (which is a CGI
script) into a PSGI application using CGI::Compile and CGI::Emulate::PSGI.
git-instaweb then runs this wrapper, using by default HTTP::Server::PSGI
standalone Perl server, via Plack::Runner.

The configuration for 'plackup' is currently embedded in generated
gitweb.psgi wrapper, instead of using httpd.conf ($conf).

To run git-instaweb with '--httpd=plackup', you need to have instaled
Plack core, CGI::Emulate::PSGI, CGI::Compile.  Those modules have to be
available for Perl scripts (which can be done for example by setting
PERL5LIB environment variable).  This is currently not documented.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Acked-by: Petr Baudis <pasky@suse.cz>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>