Check commit count against `upstream`

[webknjaz]

Previously, the git log command would run against an unresolved branch reference which might point to the wrong thing locally. This patch makes it more predictable by specifying the exact remote branch.

Fixes #155.

[ghost]

also related: gh-124

[python-cla-bot[bot]]

All commit authors signed the Contributor License Agreement.

[webknjaz]

@hugovk mind getting this in? I just encountered this again...

[hugovk]

This has been released in Cherry Picker 2.6.0: https://github.com/python/cherry-picker/releases/tag/cherry-picker-v2.6.0 🍒🚀