Improve error handling in git-rebase-update

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: http://src.chromium.org/chrome/trunk/tools/depot_tools@292444 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
Git footer fix for v8

Git footers doesn't seem to support v8, let alone know that trunk actually maps
to the candidate branch.  This should fix it.

BUG=
TBR=agable

Review URL: https://codereview.chromium.org/645183002

git-svn-id: http://src.chromium.org/chrome/trunk/tools/depot_tools@292445 4ff67af0-8c30-449e-8e8b-ad334ec8d88c