[PATCH 0 of 2] transaction descriptions

[Steve Borho]

This patch series adds a description to all transactions and stores
them in .hg/undo.desc where the rollback command can later find them.
undo.desc will contain "rev,command[,url]" where the command (one of commit,
push, pull, serve, or unbundle) is untranslated and url is optional.

The second patch adds a -n/--dry-run argument to rollback so the user
may preview the changes that will be un-done.  The url is only displayed
if --verbose is specified.