VMR sync behavior can be confusing if you make local changes

[mmitche]

If a dev makes local changes to a VMR (in a source repo dir), then attempts to sync to latest in the VMR for that repo, and there are conflicts, you get into an unresolvable situation. The patches fail (it also isn't super obvious which changes are causing the issue), and then part of the changes get staged while the remainder just stops.

We should have a better story for this which:

• Tells the user what went wrong
• Has options for resolution

Suggested options for resolution might include

• Enabling an overwrite switch (force? overwrite), staging changes (so people can see what the diff is from their HEAD). I think this should be an option no matter what.
• Merging somehow

[mmitche]

/cc @premun