update_depot_tools: Workaound svn symlink issue.

svn info doesn't like it when handed a symlink - you get an 'is not a working copy' error message. Unfortunately, the Mac uses BSD readlink, and doesn't have -f, so the subshell CD/PWD idiom must be used.

BUG=none
TEST=Create a symlink to depot_tools, put it early in your path, and do update_depot_tools. You should not get any 'not a working copy' error messages.
Review URL: http://codereview.chromium.org/6838001

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@81323 0039d316-1c4b-4281-b951-d872f2087c98

update_depot_tools

@@ -6,6 +6,10 @@
 # This script will try to sync the bootstrap directories and then defer control.
 
 base_dir=$(dirname "$0")
+if [ -L "$base_dir" ]
+then
+    base_dir=`cd "$base_dir" && pwd -P`
+fi
 
 # Test git and git --version.
 function test_git {