Revert "bot_update: allow rebasing the patch onto an older revision."

This reverts commit fa573785.

Reason for revert: broke CQ on branches.
Example: https://ci.chromium.org/p/chromium/builders/luci.chromium.try/chromium_presubmit/28220

Original change's description:
> bot_update: allow rebasing the patch onto an older revision.
>
> When applying a rebase, we normally go from an older base commit to a newer one.
>
> A ---- B ---- C ---- D -origin/master
>                \
>                 E -branch
>
> In this case, `git rebase D` would certainly work as expected. However,
> writing `git rebase B` would NOT get us to the following state:
>
> A ---- B ---- C ---- D -origin/master
>         \
>          E' -branch
>
> In fact, it would have no effect.
>
> This article http://matthew-brett.github.io/pydagogue/rebase_without_tears.html
> explains the general invocation as
> > `git rebase --onto <graft-point> <exclude-from> <include-from>`
> > If you don’t specify --onto, <graft-point> defaults to <exclude-from>
>
> So what's happening is, by writing `git rebase B` we're rebasing onto B,
> excluding commits that are in B. Commit C is not "in" B so it is kept and we're
> back to the starting point.
>
> So I suggest to change the invocation to `git rebase --onto B origin/master`,
> which rebases onto B, excluding commits that are in origin/master. This works
> more generally and allows rebasing "backwards".
>
> Bug: None
> Change-Id: I68e4d805811530b585550bc75099354fef4e9c15
> Reviewed-on: https://chromium-review.googlesource.com/904004
> Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
> Commit-Queue: Oleh Prypin <oprypin@chromium.org>

TBR=iannucci@chromium.org,tandrii@chromium.org,oprypin@chromium.org

Change-Id: I7243641d84428bf4504ee798bf31ad0afbfd4865
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: None
Reviewed-on: https://chromium-review.googlesource.com/905862
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -784,7 +784,7 @@ def apply_gerrit_ref(gerrit_repo, gerrit_ref, root, gerrit_reset,
         try:
           git('-c', 'user.name=chrome-bot',
               '-c', 'user.email=chrome-bot@chromium.org',
-              'rebase', '--onto', base_rev, 'origin/master', cwd=root)
+              'rebase', base_rev, cwd=root)
         except SubprocessFailed:
           # Abort the rebase since there were failures.
           git('rebase', '--abort', cwd=root)