Changes:
- _UploadAllPrecheck() returns a list of cls to upload that skips any empty commits in the stack.
- If the current branch is empty, we throw an error.
- UploadAllSquashed() now computes the parent to use for the squashed and cherry-pick flow.
- for cherry-pick it uses the gerrit_squash_hash of the next cl in `cls`. This means it could be the gerrit_squash_hash of the direct ancestor OR the closest non-empty ancestor branch.
- for multiple squashed commits, the first parent passed in is either:
- the closest ancestor with a gerrit_squash_hash OR the common ancestor shared with the root of the tree.
- PrepareSquashedCommit() and PrepareCherryPick() now both require a parent passed in.
Bug:1411878, b/265929888
Change-Id: I7dba289defb40ed0464eabdb7e90810353ef155f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4220412
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Joanna Wang <jojwang@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>
