gitweb: allow configurations that change with each request

gitolite's contrib/gitweb/gitweb.conf includes:

	$ENV{GL_USER} = $cgi->remote_user || "gitweb";

which is useful for setups where a user has to be authenticated
to access certain repos.  Perhaps other typical configurations
change per session in other ways, too.

v1.7.2-rc2~6 (gitweb: Move evaluate_gitweb_config out of run_request,
2010-07-05) broke such configurations for a speedup, by loading
the configuration once per FastCGI process.

Probably in the end there should be a way to specify in the
configuration whether a particular installation wants the speedup or
the flexibility.  But for now it is easier to just undo the relevant
change.

This partially reverts commit 869d58813b24c74e84c9388041eafcef40cb51e4.

Reported-by: Julio Lajara <redacted>
Analysis-by: Jakub Narebski <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>