Start all test scripts with /bin/sh.

My bash refused to run the two scripts missing a #!, and it's
better to use the same line for all the scripts.

Signed-off-by: Jason Riedy <ejr@cs.berkeley.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>

t/t9102-git-svn-deep-rmdir.sh

@@ -1,3 +1,4 @@
+#!/bin/sh
 test_description='git-svn rmdir'
 . ./lib-git-svn.sh
 

t/t9103-git-svn-graft-branches.sh

@@ -1,3 +1,4 @@
+#!/bin/sh
 test_description='git-svn graft-branches'
 . ./lib-git-svn.sh
 

t/t9200-git-cvsexportcommit.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 #
 # Copyright (c) Robin Rosenberg
 #