git-svn: avoid md5 calculation entirely if SVN doesn't provide one

There's no point in calculating an MD5 if we're not going to use
it.  We'll also avoid the possibility of there being a bug in the
Perl MD5 library not being able to handle zero-sized files.

This is a followup to 20b3d206ac,
which allows us to track repositories that do not provide MD5
checksums.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-svn.perl

@@ -2472,12 +2472,16 @@ sub close_file {
 	my $hash;
 	my $path = $self->git_path($fb->{path});
 	if (my $fh = $fb->{fh}) {
-		seek($fh, 0, 0) or croak $!;
-		my $md5 = Digest::MD5->new;
-		$md5->addfile($fh);
-		my $got = $md5->hexdigest;
-		die "Checksum mismatch: $path\n",
-		    "expected: $exp\n    got: $got\n" if (defined $exp && $got ne $exp);
+		if (defined $exp) {
+			seek($fh, 0, 0) or croak $!;
+			my $md5 = Digest::MD5->new;
+			$md5->addfile($fh);
+			my $got = $md5->hexdigest;
+			if ($got ne $exp) {
+				die "Checksum mismatch: $path\n",
+				    "expected: $exp\n    got: $got\n";
+			}
+		}
 		sysseek($fh, 0, 0) or croak $!;
 		if ($fb->{mode_b} == 120000) {
 			sysread($fh, my $buf, 5) == 5 or croak $!;