Make ReportOutputTable support hierarchical data

Rows can have children. Sorting is per level.

XML:    Rows with children have a 'children' element containing all child
        rows sequentially.
JSON:   Rows with children have a 'children' field containing a list of
        child rows.
Text:   The initial field of each row is indented wrt its parent.
CSV:    Display is flat: parent/child relationship is not represented.
Add support for fetching dependencies from gerrit
Consolidate arguments to ToDo commands
Refactor common code for changelist reports
Add change dependency output to ReportChangeList
Add --deps option to changes and all todo commands