git-svn: fix --file/-F option in commit-diff

Also, allow messages from tags to be used as
commit messages.
Signed-off-by: NEric Wong <normalperson@yhbt.net>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-svn.perl

@@ -760,7 +760,7 @@ sub commit_diff {
 		exit 1;
 	}
 	if (defined $_file) {
-		$_message = file_to_s($_message);
+		$_message = file_to_s($_file);
 	} else {
 		$_message ||= get_commit_message($tb,
 					"$GIT_DIR/.svn-commit.tmp.$$")->{msg};
@@ -1518,12 +1518,12 @@ sub get_commit_message {
 	open my $msg, '>', $commit_msg or croak $!;
 
 	chomp(my $type = `git-cat-file -t $commit`);
-	if ($type eq 'commit') {
+	if ($type eq 'commit' || $type eq 'tag') {
 		my $pid = open my $msg_fh, '-|';
 		defined $pid or croak $!;
 
 		if ($pid == 0) {
-			exec(qw(git-cat-file commit), $commit) or croak $!;
+			exec('git-cat-file', $type, $commit) or croak $!;
 		}
 		my $in_msg = 0;
 		while (<$msg_fh>) {