Git.pm: correctly handle directory name that evaluates to "false"

The repository constructor mistakenly rewrote a Directory parameter that
Perl happens to evaluate to false (e.g. "0") to ".".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

perl/Git.pm

@@ -166,11 +166,12 @@ sub repository {
 		}
 	}
 
-	if (not defined $opts{Repository} and not defined $opts{WorkingCopy}) {
-		$opts{Directory} ||= '.';
+	if (not defined $opts{Repository} and not defined $opts{WorkingCopy}
+		and not defined $opts{Directory}) {
+		$opts{Directory} = '.';
 	}
 
-	if ($opts{Directory}) {
+	if (defined $opts{Directory}) {
 		-d $opts{Directory} or throw Error::Simple("Directory not found: $!");
 
 		my $search = Git->repository(WorkingCopy => $opts{Directory});