Here is an explanation of the changes in each module:

* git_cl.py -
IF a local branch is being tracked AND a CL has been uploaded there THEN use the CL's issue number and latest patchset as a dependency.

* upload.py -
Uploads the patchset dependency, if it exists, to Rietveld (Rietveld will be able to parse this when https://codereview.chromium.org/1155513002/ lands).

* rietveld.py -
Adds utility methods to get patchset dependencies from the new Rietveld endpoint (the endpoint will exist when https://codereview.chromium.org/1155513002/ lands).

* apply_issue.py -
If CL3 depends on CL2 which in turn depends on CL1 then apply_issue will gather a list of all issues and patchsets to apply (Eg: [CL1:PS1, CL2:PS1, CL3:PS2]).
apply_issue will then loop over the list applying each dependency.
Note: The apply_issue.py diff looks much worse than it is. Please see my comment in
https://codereview.chromium.org/1149653002/diff/260001/apply_issue.py#oldcode169


Tested end-to-end using a test Git repository (https://skia.googlesource.com/skiabot-test/) and the following CLs created in my test Rietveld instance:
* https://skia-codereview-staging.appspot.com/931002  ('Branch1 CL')
* https://skia-codereview-staging.appspot.com/5001001 ('Branch2 CL')
* https://skia-codereview-staging.appspot.com/9881001 ('Branch3 CL')
* https://skia-codereview-staging.appspot.com/3951001 ('Branch3.1 CL')
Opt into the new UI and observe the new 'Depends on Patchset' and 'Dependent Patchsets' sections in the above CLs.


Design doc is here: https://docs.google.com/document/d/1KZGFKZpOPvco81sYVRCzwlnjGctup71RAzY0MSb0ntc/edit#heading=h.6r6lt4tsvssw

BUG=502255

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=295778

Review URL: https://codereview.chromium.org/1149653002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@295799 0039d316-1c4b-4281-b951-d872f2087c98