Fix a crash when having HEAD detached

With HEAD detached, the parseSymbolicReference code would find an OID as an unresolvedTarget, which would result in a nil branchRef, thus a nil _headOID, and a subsequent crash when trying to add this nil OID to a set "somewhere".

Simplify all that by just using whatever our underlying HEAD reference says.

Author: tiennou

Classes/git/PBGitRepository.h

@@ -124,7 +124,6 @@ typedef enum branchFilterTypes {
 - (PBGitRevSpecifier*) addBranch: (PBGitRevSpecifier*) rev;
 - (BOOL)removeBranch:(PBGitRevSpecifier *)rev;
 
-- (GTReference*) parseSymbolicReference:(NSString*) ref;
 - (BOOL) revisionExists:(NSString*) spec;
 
 - (void) forceUpdateRevisions;

Classes/git/PBGitRepository.m

@@ -273,15 +273,23 @@ - (void) lazyReload
 
 - (PBGitRevSpecifier *)headRef
 {
-	if (_headRef)
+	if (_headRef && _headOID)
 		return _headRef;
 
-	GTReference *branchRef = [self parseSymbolicReference: @"HEAD"];
-	if (branchRef && [branchRef.name hasPrefix:@"refs/heads/"])
-		_headRef = [[PBGitRevSpecifier alloc] initWithRef:[PBGitRef refFromString:branchRef.name]];
-	else
-		_headRef = [[PBGitRevSpecifier alloc] initWithRef:[PBGitRef refFromString:@"HEAD"]];
+	NSError *error = nil;
+	GTReference *headRef = [self.gtRepo headReferenceWithError:&error];
+	if (!headRef) {
+		PBLogError(error);
+		return nil;
+	}
 
+	GTReference *branchRef = [headRef resolvedReferenceWithError:&error];
+	if (!branchRef) {
+		PBLogError(error);
+		return nil;
+	}
+
+	_headRef = [[PBGitRevSpecifier alloc] initWithRef:[PBGitRef refFromString:branchRef.name]];
 	_headOID = branchRef.OID;
 
 	return _headRef;
@@ -1108,16 +1116,4 @@ - (BOOL)revisionExists:(NSString *)spec
 	return [self.gtRepo lookUpObjectByRevParse:spec error:nil] != nil;
 }
 
-- (GTReference *)parseSymbolicReference:(NSString*) reference
-{
-	GTReference *gtRef = [self.gtRepo lookUpReferenceWithName:reference error:nil];
-	if (!gtRef) return nil;
-	id target = gtRef.unresolvedTarget;
-	if ([target isKindOfClass:[GTReference class]]) {
-		NSString *ref = ((GTReference *)target).name;
-		if ([ref hasPrefix:@"refs/"]) return target;
-	}
-    return nil;
-}
-
 @end