Add "git cl cherry-pick" for cherry picking a chain of CLs via Gerrit

Users who want to upload multiple cherry picks usually run "git
cherry-pick" locally, multiple times. Gerrit does not recognize
these changes as cherry picks and neither do other services that
query cherry pick info from Gerrit, e.g. rubber stamper.

For Gerrit to identify a change as a true cherry pick, you need to
use their Cherry Pick Revision REST API endpoint. This new command
uses it to create a chain of cherry pick CLs recognized by Gerrit.

Bug: b/341792235
Change-Id: I4ba75da3901f6ea68c1debd65820e802da681798
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5756161
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>

gerrit_util.py

@@ -1326,6 +1326,14 @@ def RestoreChange(host, change, msg=''):
     return ReadHttpJsonResponse(conn)
 
 
+def RebaseChange(host, change, base=None):
+    """Rebases a change."""
+    path = f'changes/{change}/rebase'
+    body = {'base': base} if base else {}
+    conn = CreateHttpConn(host, path, reqtype='POST', body=body)
+    return ReadHttpJsonResponse(conn)
+
+
 def SubmitChange(host, change):
     """Submits a Gerrit change via Gerrit."""
     path = 'changes/%s/submit' % change
@@ -1387,12 +1395,14 @@ def DeletePendingChangeEdit(host, change):
     ReadHttpResponse(conn, accept_statuses=[204, 404])
 
 
-def CherryPick(host, change, destination, revision='current'):
+def CherryPick(host, change, destination, revision='current', message=None):
     """Create a cherry-pick commit from the given change, onto the given
     destination.
     """
     path = 'changes/%s/revisions/%s/cherrypick' % (change, revision)
     body = {'destination': destination}
+    if message:
+        body['message'] = message
     conn = CreateHttpConn(host, path, reqtype='POST', body=body)
     return ReadHttpJsonResponse(conn)