update_depot_tools: Workaound svn symlink issue.
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: bd64dd6fa6f3f0ed0c0666d1018379882b742947@81323 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools