URL value is dependent on the local directory. The current directory would
affect Base URL on Rietveld otherwise.

Do not use Repository Root instead of URL since git svn clone foo -T bar
wouldn't be correctly based.

Update RunCommand() to be able to pass cwd argument.

TEST=git cl upload is broken if I break this code.
BUG=none

Review URL: http://codereview.chromium.org/6591093

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@76791 0039d316-1c4b-4281-b951-d872f2087c98