gitweb: require $ENV{'GITWEB_CONFIG'}

gitweb: require $ENV{'GITWEB_CONFIG'}

With this patch it is possible to use gitweb.perl for developing by
loading the configuration from $GITWEB_CONFIG.  This might also be
useful for normal usage of gitweb.

Example:

    % cat cfg
    $GIT = '/usr/bin/git';
    $projectroot = '/home/matled/src/git';
    $projects_list = '/home/matled/src/git/git/gitweb/list';
    % cat run
    #!/bin/sh
    export GATEWAY_INTERFACE="CGI/1.1"
    export HTTP_ACCEPT="*/*"
    export REQUEST_METHOD="GET"
    export GITWEB_CONFIG='./cfg'
    export QUERY_STRING=""$1""
    exec ./gitweb.perl
    % time ./run p=git/.git > /dev/null

This makes it easy to check for warnings and do performance tests
after changes, you can also pipe this to lynx -dump -force-html
/dev/stdin to get more than just html.

This also documents the original patch adding require $GITWEB_CONFIG.

Signed-off-by: Matthias Lederhofer <redacted>
Signed-off-by: Junio C Hamano <redacted>