git cl upload gets a new flag --edit-description, which allows user to
edit git commit message. It only works when updating existing CL.

Bug: 739928
Change-Id: Ia501dc31f32e1887a937d5df39aed03745376827
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2020591Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>