git cl cherry-pick: Use base parameter to fix identical tree errors

The `git cl cherry-pick` command previously created chained CLs by
first cherry-picking a change onto the destination branch tip and then
rebasing the result onto the parent CL created in the previous step.

This approach failed when a sequence of cherry-picks resulted in an
intermediate state having an identical tree compared to its intended
base (e.g., commit 1 changes X->Y, commit 2 changes Y->X). Gerrit
would reject the second cherry-pick with an "identical tree" error
because the rebase is done after the cherry pick.

This change modifies the process to use the `base` parameter of the
Gerrit `cherrypick` REST API endpoint.

Changes:
- Modify `gerrit_util.CherryPick` to accept and pass an optional `base`
  commit hash in the API request body.
- Update `git_cl.CMDcherry_pick`:
  - Before each cherry-pick operation in the loop, fetch the commit hash
    of the latest patchset from the previously processed parent CL.
  - Pass this commit hash as the `base` parameter to `gerrit_util.CherryPick`.
  - Remove the subsequent, now redundant, call to `gerrit_util.RebaseChange`.

This ensures the correct parent commit is specified during the
cherry-pick operation itself, allowing Gerrit to handle the chaining
correctly and avoid failures caused by identical tree states in
intermediate steps.

Bug: 408388488
Change-Id: I84066d65bd6bb127b253bee6564dd0622148a0e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6433112
Commit-Queue: Gennady Tsitovich <gtsitovich@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

gerrit_util.py

@@ -1518,7 +1518,12 @@ def DeletePendingChangeEdit(host, change):
     ReadHttpResponse(conn, accept_statuses=[204, 404])
 
 
-def CherryPick(host, change, destination, revision='current', message=None):
+def CherryPick(host,
+               change,
+               destination,
+               revision='current',
+               message=None,
+               base=None):
     """Create a cherry-pick commit from the given change, onto the given
     destination.
     """
@@ -1526,6 +1531,8 @@ def CherryPick(host, change, destination, revision='current', message=None):
     body = {'destination': destination}
     if message:
         body['message'] = message
+    if base:
+        body['base'] = base
     conn = CreateHttpConn(host, path, reqtype='POST', body=body)
 
     # If a cherry pick fails due to a merge conflict, Gerrit returns 409.