After installing gitweb on an Ubuntu 12.04 (64 bit) server and changing the variable $projectroot in /usr/share/gitweb/gitweb.cgi to any directory other than /var/cache/git, projects are still searched in /var/cache/git.
This happens with Ubuntu’s gitweb package (1.7.9.5) and also cloning Git’s repository and compiling the latest stable branch (1.8.0.1).
The problem can be also experienced as a "404 - no projects found" on Apache’s gitweb site, because projects would be in a directory different to /var/cache/git.
In Ubuntu the file /usr/share/gitweb/gitweb.cgi shouldn’t be edited. The $projectroot variable should be set in /etc/gitweb.conf.
Thanks to the Git’s mailing list for the help.