Support specifying the parent for a cherry-pick in the REST API

When cherry-picking a merge, the REST API always selected the first
parent as reference. Sometimes users of the API want to choose a
specific parent just as they could on the command line using 'git
cherry-pick -m'. If the parent parameter isn't specified, it defaults
to 1.

Change-Id: I61dd0d9bee4a3c97354d52a18c66ae3567ab79fb

Documentation/rest-api-changes.txt

diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index ae02475..32e93d3 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -4860,11 +4860,13 @@
 === CherryPickInput
 The `CherryPickInput` entity contains information for cherry-picking a change to a new branch.
 
-[options="header",cols="1,6"]
+[options="header",cols="1,^1,5"]
 |===========================
-|Field Name    |Description
-|`message`     |Commit message for the cherry-picked change
-|`destination` |Destination branch
+|Field Name         ||Description
+|`message`          ||Commit message for the cherry-picked change
+|`destination`      ||Destination branch
+|`parent`           |optional, defaults to 1|
+Number of the parent relative to which the cherry-pick should be considered.
 |===========================
 
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