fix(ci): use three-dot diff to detect only PR changes in build workflow (opendatahub-io#2876)

* fix(tests): update test_get_build_directory to expect Python 3.12

The test was checking for ubi9-python-3.11 but the actual directory uses
ubi9-python-3.12. Update the assertion to match the current codebase state.

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(ci): use three-dot diff to detect only PR changes in build workflow

The build workflow was using a two-dot git diff which compares the complete
state of two commits. When main advances with new commits after a PR branch
is created, the two-dot diff shows ALL differences between the branches,
including files that changed in main but not in the PR.

This caused false positive build triggers. For example, PR opendatahub-io#2874 only changed
scripts/README.md and scripts/sbom_analyze.py, but the CI detected 20 changed
files (including all pylock.toml files that were updated in main by PR opendatahub-io#2873
after PR opendatahub-io#2874 was created), triggering unnecessary builds for all workbench
images.

Fix: Change from two-dot diff (A B) to three-dot diff (A...B) which shows
changes from the merge-base of A and B to commit B. This correctly identifies
only the changes introduced by the PR, regardless of how much main has
advanced.

Fixes: opendatahub-io#2875
Co-authored-by: Cursor <cursoragent@cursor.com>

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: jiridanek

ci/cached-builds/gha_pr_changed_files.py

@@ -22,9 +22,12 @@ def get_github_token() -> str:
 def list_changed_files(from_ref: str, to_ref: str) -> list[str]:
     logging.debug("Getting list of changed files from git diff")
 
+    # Use three-dot diff to show changes from merge-base to to_ref
+    # This correctly shows only changes introduced by the PR, regardless of how much from_ref has advanced
+    # See: https://github.com/opendatahub-io/notebooks/issues/2875
     # https://github.com/red-hat-data-services/notebooks/pull/361: add -- in case to_ref matches a file name in the repo
     files = subprocess.check_output(
-        ["git", "diff", "--name-only", from_ref, to_ref, "--"], encoding="utf-8"
+        ["git", "diff", "--name-only", f"{from_ref}...{to_ref}", "--"], encoding="utf-8"
     ).splitlines()
 
     logging.debug(f"Determined {len(files)} changed files: {files[:100]} (..., printing up to 100 files)")
@@ -157,7 +160,7 @@ def test_list_changed_files(self):
 
     def test_get_build_directory(self):
         directory = get_build_directory("rocm-jupyter-pytorch-ubi9-python-3.12")
-        assert directory == "jupyter/rocm/pytorch/ubi9-python-3.11"
+        assert directory == "jupyter/rocm/pytorch/ubi9-python-3.12"
 
     def test_get_build_dockerfile(self):
         dockerfile = get_build_dockerfile("rocm-jupyter-pytorch-ubi9-python-3.12")