Only attempt to restore Pages if we have a backup

[lildude]

If a GitHub Enterprise appliance doesn't have any Pages generated or Pages have always been disabled on the appliance, there won't be a "Pages" directory to backup resulting in no "pages" directory in the backup when using rsync.

This causes a problem when restoring as ghe-restore-pages-rsync doesn't take this into account and fails with:

Starting rsync restore of ghe.vm from snapshot 20140821T133259
Connect ghe.vm OK (v11.10.343)
Enabling maintenance mode on ghe.vm ...
Restoring Git repositories ...
Restoring GitHub Pages ...
rsync: link_stat "/path/to/backup-utils/data/20140821T133259/pages/." failed: No such file or directory (2)
rsync error: some files could not be transferred (code 23) at /SourceCache/rsync/rsync-42/rsync/main.c(992) [sender=2.6.9]

This PR corrects this so a restore of Pages over rsync will only be attempted if there is a backup to restore.

[rtomayko]

Looks good. Added tests.

I'm going to merge this to master manually. I made the default branch "stable", which I'd like to always point to the latest release. Unfortunately this means PRs are automatically based there as well. I'm going to update the readme to suggest git clone -b stable ... and switch the default branch back to master to avoid this in the future.

[rtomayko]

Merged to master. Closing....