gitweb: fill in gitweb configuration by Makefile

Generate gitweb/gitweb.cgi to reduce the need to patch gitweb.cgi
by the end user.  The GIT installation directory is already known
by the Makefile, and can be inserted directly into gitweb.
All other gitweb configuration parameters can now be specified by
providing GITWEB_* variables while building GIT.  These are described
in gitweb/README.

Signed-off-by: Martin Waitz <redacted>
Signed-off-by: Junio C Hamano <redacted>