git-svn: Fix git svn log --show-commit

git svn log --show-commit had no tests and, consequently, no attention
by the author of

b1b4755 (git-log: put space after commit mark, 2011-03-10)

who kept git svn log working only without --show-commit.

Introduce a test and fix it.

Reported-by: Bernt Hansen <bernt@norang.ca>
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-svn.perl b/git-svn.perl
index bf0451b..da3fea8 100755
--- a/git-svn.perl
+++ b/git-svn.perl
@@ -5752,7 +5752,7 @@
 	my (@k, $c, $d, $stat);
 	my $esc_color = qr/(?:\033\[(?:(?:\d+;)*\d*)?m)*/;
 	while (<$log>) {
-		if (/^${esc_color}commit (- )?($::sha1_short)/o) {
+		if (/^${esc_color}commit (?:- )?($::sha1_short)/o) {
 			my $cmt = $1;
 			if ($c && cmt_showable($c) && $c->{r} != $r_last) {
 				$r_last = $c->{r};