Move mercurial mirror script to the infrastructure directory; it's not
really "nightly" scripting.

Also, the crontab files were already moved to the infrastructure
directory; forgot to commit their removals from the "nightly"
directory.
Oops -- the last commit moved the "update www.open-mpi.org" script
(i.e., the script that fires via cron every 15 mins to "git pull" from
the github ompi-www repo).

This commit moves the mercurial mirroring script to the
"infrastructure" directory.
Trivial defensive programming/style update: use {}, even for 1-line blocks.