GitCommit: Add a 'removeRef' function

The ref functionality is somewhat weird right now, in that
some parts of the code require a nil array when there are no
refs. So, we add a good removeRef accessor to keep track of this

PBGitCommit.h

@@ -27,6 +27,7 @@
 - initWithRepository:(PBGitRepository*) repo andSha:(NSString*) sha;
 
 - (void)addRef:(id)ref;
+- (void)removeRef:(id)ref;
 
 @property (copy) NSString* sha;
 @property (copy) NSString* subject;

PBGitCommit.m

@@ -70,6 +70,16 @@ - (void)addRef:(id)ref
 		[self.refs addObject:ref];
 }
 
+- (void)removeRef:(id)ref
+{
+	if (!self.refs)
+		return;
+
+	[refs removeObject:ref];
+	if ([refs count] == 0)
+		refs = NULL;
+}
+
 + (BOOL)isSelectorExcludedFromWebScript:(SEL)aSelector
 {
 	return NO;

PBRefController.m

@@ -88,10 +88,7 @@ - (BOOL)tableView:(NSTableView *)aTableView
 		return NO;
 
 	[dropCommit addRef:ref];
-
-	[oldCommit.refs removeObject:ref];
-	if ([oldCommit.refs count] == 0)
-		oldCommit.refs = NULL;
+	[oldCommit removeRef:ref];
 
 	[commitController rearrangeObjects];
 	[aTableView needsToDrawRect:[aTableView rectOfRow:oldRow]];