gcl change will look for unmodified files from the current directory and below
only.
Add shell out are cwd=checkout's root.
Fix try/finally handlers.
Remove extraneous os.chdir() calls since cwd is now correctly set.

TBR=dpranke@chromium.org
BUG=
TEST=


Review URL: http://codereview.chromium.org/8994004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@115027 0039d316-1c4b-4281-b951-d872f2087c98