upload currently overwrites any external changes, e.g. edits/rebases
in Gerrit. Check for these on upload and provide the ability to
incorporate those into the CL.
Bug: 1382528
Change-Id: Id32bf8804c4cdeb12b6a5a7cf206e033bbccd453
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3975855
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>