Don't allow drag-n-drop for the checked out branch.

Moving the checked out branch would cause odd things to happen to git's index.

Author: brotherbard

PBRefController.m

@@ -294,6 +294,10 @@ - (BOOL)tableView:(NSTableView *)tv writeRowsWithIndexes:(NSIndexSet *)rowIndexe
 
 	if (index == -1)
 		return NO;
+
+	PBGitRef *ref = [[[cell objectValue] refs] objectAtIndex:index];
+	if ([[[historyController.repository headRef] ref] isEqualToRef:ref])
+		return NO;
 
 	NSData *data = [NSKeyedArchiver archivedDataWithRootObject:[NSArray arrayWithObjects:[NSNumber numberWithInt:row], [NSNumber numberWithInt:index], NULL]];
 	[pboard declareTypes:[NSArray arrayWithObject:@"PBGitRef"] owner:self];