git-svn: call 'fatal' correctly in set-tree

When doing a set-tree and there is no revision to commit to, the following unrelated error message is displayed: "Undefined subroutine &Git::SVN::fatal called at /opt/local/libexec/git-core/git-svn line 2575." The following patch fixes the problem and allows the real error message to be shown.
Signed-off-by: NLuc Heinrich <luc@honk-honk.com>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-svn.perl

@@ -2571,7 +2571,7 @@ sub set_tree {
 	my ($self, $tree) = (shift, shift);
 	my $log_entry = ::get_commit_entry($tree);
 	unless ($self->{last_rev}) {
-		fatal("Must have an existing revision to commit");
+		::fatal("Must have an existing revision to commit");
 	}
 	my %ed_opts = ( r => $self->{last_rev},
 	                log => $log_entry->{log},