Issue #2491: Add --dry-run flag to "svn update" client command

[Julian Foad <ju...@btopenworld.com>]

Markus Schaber wrote (in thread "controversial issues in the tracker"):
> * 2491 Add --dry-run flag to "svn update" client command
> http://subversion.tigris.org/issues/show_bug.cgi?id=2491
> 
> There already was a patch submitted by Arwin Arni 2011 and discussed on the 
> list, but it was not applied due to objections.
> 
> The first controversial issue was of conceptual nature: This functionality 
> somehow duplicates the semantics of "svn status -u" - on the other 
> hand, "svn status -u" is known to not exactly give the desired 
> semantics in some cases - especially, it can only tell about the existence of 
> incoming changes, but not whether they're actually produce a conflict.
> 
> The second issue was more of an implementation issue - the patch spreads a lot 
> of if()-branches in the code. Some developers would have preferred cleaner code 
> by creating a separate editor driver. However, this will result in a bunch of 
> duplicate code, which needs to be kept in sync (or the "--dry-run" 
> mode will not match what the real update will perform. There is already a 
> precedence for merge --dry-run.
> 
> See also the lengthy discussion at 
> http://svn.haxx.se/dev/archive-2011-03/index.shtml#63
> 
> My personal opinion is that it is an useful feature, so we should try to agree 
> on a way of implementing it. On the other hand, given that the provided patch 
> does not cover some corner cases, it may actually not be "bite-sized". 
> :-)

I think we *should* have a dry-run mode but it's not simple to do, so I would leave the issue open.

(Even better than a dry-run mode would be the ability to undo after running an update -- but that requires something like "stash" support which is not available yet.)

- Julian