genindex.sh: sort the project list

genindex.sh: sort the project list

In order for gitweb.cgi to properly pick up forks when using a
project list file, the forks must come AFTER the forkee.  Currently
this would always be the case since the fork must be created after
the forkee and therefore it would appear later in the group file.

However, if we start reusing unused project numbers (because some
project has been removed) and continue to maintain the group file
in project number order, the fork could appear before the forkee.

Guarantee this doesn't happen in the project list by sorting the
output.  Note that apparently glibc has a bug where sorting the
list with a UTF-8 locale active causes '.' to sort after '/' which
would break forks since we require foo.git to appear in the list
before foo/fork.git.  Fortunately explicitly setting LC_ALL=C
works around this problem.

We are careful to sort the list before applying the '+' -> '%2B'
transformation.