Using reset --soft doesn't require touching the working copy, so it should be faster and not require rebuilds.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/6575031

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@77267 0039d316-1c4b-4281-b951-d872f2087c98