[PATCH] Add --dry-run flag to "svn update" client command

Hi All,

In my effort to understand the delta editor API, I took it upon myself
to try and implement the --dry-run flag for "svn update".
With Kamesh's guidance, I think I've managed to pull it off.

Here is the relevant Issue.
http://subversion.tigris.org/issues/show_bug.cgi?id=2491

I have attached a log message and a patch that implements --dry-run for
update.

Currently, externals are handled inside
subversion/libsvn_client/externals.c by running checkout/switch. For a
dry-run update to mimic a real update, the notifications have to be the
same. Since some of these notifications are generated by the above
mentioned checkout/switch runs, I have to implement dry-run for them
also. I'll take this up as a follow-up exercise. Now, the dry-run will
simply ignore any externals in the working copy.

Please review this and share your thoughts.

Regards,
Arwin Arni