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

git_common.py

@@ -497,7 +497,7 @@ def parse_commitrefs(*commitrefs):
     raise BadCommitRefException(commitrefs)
 
 
-RebaseRet = collections.namedtuple('RebaseRet', 'success message')
+RebaseRet = collections.namedtuple('RebaseRet', 'success stdout stderr')
 
 
 def rebase(parent, start, branch, abort=False):
@@ -521,11 +521,11 @@ def rebase(parent, start, branch, abort=False):
     if TEST_MODE:
       args.insert(0, '--committer-date-is-author-date')
     run('rebase', *args)
-    return RebaseRet(True, '')
+    return RebaseRet(True, '', '')
   except subprocess2.CalledProcessError as cpe:
     if abort:
       run('rebase', '--abort')
-    return RebaseRet(False, cpe.stdout)
+    return RebaseRet(False, cpe.stdout, cpe.stderr)
 
 
 def remove_merge_base(branch):

git_rebase_update.py

@@ -130,7 +130,8 @@ def rebase_branch(branch, parent, start_hash):
   if git.hash_one(parent) != start_hash:
     # Try a plain rebase first
     print 'Rebasing:', branch
-    if not git.rebase(parent, start_hash, branch, abort=True).success:
+    rebase_ret = git.rebase(parent, start_hash, branch, abort=True)
+    if not rebase_ret.success:
       # TODO(iannucci): Find collapsible branches in a smarter way?
       print "Failed! Attempting to squash", branch, "...",
       squash_branch = branch+"_squash_attempt"
@@ -148,14 +149,19 @@ def rebase_branch(branch, parent, start_hash):
         git.rebase(parent, start_hash, branch)
       else:
         # rebase and leave in mid-rebase state.
-        git.rebase(parent, start_hash, branch)
+        # This second rebase attempt should always fail in the same
+        # way that the first one does.  If it magically succeeds then
+        # something very strange has happened.
+        second_rebase_ret = git.rebase(parent, start_hash, branch)
+        assert(not second_rebase_ret.success)
         print "Failed!"
         print
-        print "Here's what git-rebase had to say:"
-        print squash_ret.message
+        print "Here's what git-rebase (squashed) had to say:"
         print
+        print squash_ret.stdout
+        print squash_ret.stderr
         print textwrap.dedent(
-        """
+        """\
         Squashing failed. You probably have a real merge conflict.
 
         Your working copy is in mid-rebase. Either: