`Tools/build/compute-changes.py` looks at the wrong branches

[JelleZijlstra]

Bug report

Bug description:

Tools/build/compute-changes.py assumes that both of the branches being compared are on the origin origin. This is usually not going to be the case for PR branches. If the PR branch happens to be called main, this can lead to no tests being run, as happened on #131419.

cc @AA-Turner

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• GH-133410: change detection #133409
• GH-133410: Use commit hashes for change detection #133416
• [3.12] GH-133410: Use commit hashes for change detection (gh-133416) #133426
• [3.13] GH-133410: Use commit hashes for change detection (gh-133416) #133427
• gh-133410: Fix PR detection in build workflow #133671
• [3.13] gh-133410: Fix PR detection in build workflow (GH-133671) #134054
• [3.14] gh-133410: Fix PR detection in build workflow (GH-133671) #134055
• [3.12] gh-133410: Fix PR detection in build workflow (GH-133671) #134057