Force the medium pretty format on calls to git log

If a user has customized format.pretty in config, git-svn rebase fails with:

	Unable to determine upstream SVN information from working tree history

because the command expects to read the commit log in the default format.

This fixes the command to explicitly ask for the format it wants to read
from.

Signed-off-by: Pedro Melo <melo@simplicidade.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-svn.perl

@@ -1120,7 +1120,7 @@ sub cmt_metadata {
 
 sub working_head_info {
 	my ($head, $refs) = @_;
-	my @args = ('log', '--no-color', '--first-parent');
+	my @args = ('log', '--no-color', '--first-parent', '--pretty=medium');
 	my ($fh, $ctx) = command_output_pipe(@args, $head);
 	my $hash;
 	my %max;