t9104: make hash size independent

The size of a record in the database used by git svn is four bytes plus
the length of the binary hash.  Instead of hard-coding 24, compute this
value based on the size of the hash in use.
Signed-off-by: Nbrian m. carlson <sandals@crustytoothpaste.net>
Acked-by: NEric Wong <e@80x24.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t9104-git-svn-follow-parent.sh

@@ -161,6 +161,7 @@ test_expect_success "track initial change if it was only made to parent" '
 	'
 
 test_expect_success "follow-parent is atomic" '
+	record_size=$(($(test_oid rawsz) + 4)) &&
 	(
 		cd wc &&
 		svn_cmd up &&
@@ -186,7 +187,7 @@ test_expect_success "follow-parent is atomic" '
 	mkdir -p "$GIT_DIR"/svn/refs/remotes/flunk@18 &&
 	rev_map=$(cd "$GIT_DIR"/svn/refs/remotes/stunk && ls .rev_map*) &&
 	dd if="$GIT_DIR"/svn/refs/remotes/stunk/$rev_map \
-	   of="$GIT_DIR"/svn/refs/remotes/flunk@18/$rev_map bs=24 count=1 &&
+	   of="$GIT_DIR"/svn/refs/remotes/flunk@18/$rev_map bs=$record_size count=1 &&
 	rm -rf "$GIT_DIR"/svn/refs/remotes/stunk &&
 	git svn init --minimize-url -i flunk "$svnrepo"/flunk &&
 	git svn fetch -i flunk &&