gitweb_config.perl: prevent gitweb from having a stale cwd

gitweb_config.perl: prevent gitweb from having a stale cwd

The web server starts up gitweb with the current working directory
(cwd) set to the directory the gitweb.cgi script is located in.
Gitweb itself has no requirement for any particular cwd and so it
leaves it unchanged.

However, if gitweb is running in fast CGI mode, the gitweb process
could persist for some time potentially holding on to a cwd that has
become invalid as a result of a subsequent update install.

To prevent this from happening, do a "cd /" at the end of the gitweb
configuration script.

Note that it's not really appropriate for gitweb itself to do the
"cd /" as the config script may depend on the cwd being the directory
the script is located in.  So the proper place to do "cd /" really is
in the config script.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>