git-instaweb: Change how gitweb.psgi is made runnable as standalone app

git-instaweb: Change how gitweb.psgi is made runnable as standalone app

According to blog post "FindBin, __FILE__, $0 and PSGI woes"

  http://bulknews.typepad.com/blog/2011/02/findbin-__file__-0-and-psgi-woes.html

by Tatsuhiko Miyagawa, using 'if (__FILE__ eq $0)' in .psgi code
(to check if script was run from command line), is not supposed to work
since Plack 0.9971.

Replace it with one of proposed solutions; while at it return $app
explicitely, rather than implicitely by being a last expression.

This affects 'plackup' web server.

While at it cleanup whitespace.

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