Improve the way workspace conflicts that occur during Update are handled.

Previous approaches have involved interactive solutions (pop up an external file merger), or adding conflicting files in the workspace (ConflictFixupPolicy, MergeViaWorkingDir), to be cleaned up by the user later.

Here we propose a different approach, extending 'mtn conflicts' to handle workspace conflicts.

The process is to first identify all conflicts in a proposed update, and then provide a mechanism to specify resolutions for each of them. After all resolutions are specified, the update can take place.

see also WorkspaceConflicts see also NonMergeConflicts