git-refresh: fix abort by cd failure

* tool/git-refresh: try `cd -P` in a subshell, because Solaris10
  sh dies if it fails, not only the command.
  [ruby-dev:50074] [Bug #13433]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58344 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/git-refresh

@@ -1,7 +1,7 @@
 #!/bin/sh
 set -e
 
-if cd -P . 2>/dev/null; then
+if (cd -P .) 2>/dev/null; then
     CHDIR='cd -P'
 else
     CHDIR='cd'