Don't discard stderr from failed rebase operations
I had an issue where stdout of the failed rebase was
empty but stderr contained:
First, rewinding head to replay your work on top of it...
Dirty index: cannot apply patches (dirty: internal_gyp
third_party/html_office).

Also, in my case the second rebase was actually succeeding
for some reason, which is clearly no expected, so assert
in this case.

BUG=410339

Review URL: https://codereview.chromium.org/645763002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@292444 0039d316-1c4b-4281-b951-d872f2087c98