PBGitRepository: Don't return a ref if we error out

This fixes the -parseReference: method to return nil if
there is an error in the git call. This also simplifies
our commit controller.

PBGitCommitController.m

@@ -78,14 +78,9 @@ - (NSString *) parentTree
 {
 	NSString *parent = amend ? @"HEAD^" : @"HEAD";
 
-	NSString *a = [repository parseReference:@"HEAD^"];
-
-	// TODO: parseReference should exit nil if it errors out. For
-	// now, compare to "HEAD"
-	if ([a isEqualToString:parent]) {
+	if (![repository parseReference:parent])
 		// We don't have a head ref. Return the empty tree.
 		return @"4b825dc642cb6eb9a060e54bf8d69288fbee4904";
-	}
 
 	return parent;
 }

PBGitRepository.m

@@ -368,9 +368,14 @@ - (NSString*) outputForArguments:(NSArray *)arguments inputString:(NSString *)in
 							   retValue: ret];
 }
 
-- (NSString*) parseReference:(NSString *)reference
+- (NSString *)parseReference:(NSString *)reference
 {
-	return [self outputForArguments:[NSArray arrayWithObjects: @"rev-parse", reference, nil]];
+	int ret = 1;
+	NSString *ref = [self outputForArguments:[NSArray arrayWithObjects: @"rev-parse", @"--verify", reference, nil] retValue: &ret];
+	if (ret)
+		return nil;
+
+	return ref;
 }
 
 - (NSString*) parseSymbolicReference:(NSString*) reference