Prevent drag and drop of tags and remote branches

Moving an annotated tag would convert it to a lightweight tag and lose the tags message.

Given that tags are meant to represent a fixed point in the history and remote branches represent the state of the remote I'm disabling moving them.

Author: brotherbard

PBRefController.m

@@ -305,6 +305,9 @@ - (BOOL)tableView:(NSTableView *)tv writeRowsWithIndexes:(NSIndexSet *)rowIndexe
 		return NO;
 
 	PBGitRef *ref = [[[cell objectValue] refs] objectAtIndex:index];
+	if ([ref isTag] || [ref isRemoteBranch])
+		return NO;
+
 	if ([[[historyController.repository headRef] ref] isEqualToRef:ref])
 		return NO;