Allow to set in environment the numer of backups kept around by hot-backup.py and add --num-backups option which can be used to overwrite value obtained from environment.
Allow to set in environment the numer of backups kept around by hot-backup.py
and add --num-backups option which can be used to overwrite value obtained from
environment.

* tools/backup/hot-backup.py.in
  (num_backups): Document that initialization is just a default. Try to
  initialize using SVN_HOTBACKUP_BACKUPS_NUMBER environment variable.
  (usage): Document new option.
  (option parsing section): Parse it.

Patch by: kfogel
          me
Approved by: kfogel

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion