Changed the behaviour of '--transitive' in gclient.py to use revision instead of timestamp for identical repositories.

Here's some background why we need this:
We discovered that google code defines the timestamp of a revision to be the time when a commit was started
rather than when it was finished (apache subversion takes the timestamp when the commit transaction is finished).
This can result in a situation where revision R(i-1) has a higher timestamp than Ri.
See bug: https://code.google.com/p/support/issues/detail?id=30419

When using 'gclient --transitive' we effectively do date-based checkouts.
If a parent has a dependency (without a ...@revision) and that dependency lives in the same repository as the parent does
we'd like to checkout the exact same revision as the parent (if we do a date-based checkout as we do now the google code
bug can result in a situation where we don't get the same revision).

Review URL: https://chromiumcodereview.appspot.com/13814012

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@194852 0039d316-1c4b-4281-b951-d872f2087c98