[llvm] add logging to pipeline generation (PR #68983)

[Mikhail Goncharov via llvm-commits]

https://github.com/metaflow updated https://github.com/llvm/llvm-project/pull/68983

>From 8988fb5d3c4b0201d71f92614a00d154b12f28bf Mon Sep 17 00:00:00 2001
From: Mikhail Goncharov <goncharov.mikhail at gmail.com>
Date: Mon, 16 Oct 2023 11:14:12 +0200
Subject: [PATCH] [ci] pull main branch before getting a diff

we tried to generate a full diff against main in
ec9d80ec43f5761a34c4a785c67d9e7d21ec8bda but it resulted in wrong diffs
[citation needed]. It seems that the issue was that 'main' was not
updated after agent restart and diff main...HEAD kept growing.

Not enabling diff main...HEAD just yet and will check logs for new PRs
first.
---
 .ci/generate-buildkite-pipeline-premerge | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/.ci/generate-buildkite-pipeline-premerge b/.ci/generate-buildkite-pipeline-premerge
index e389df431505bd1..1028c08e20fcd0a 100755
--- a/.ci/generate-buildkite-pipeline-premerge
+++ b/.ci/generate-buildkite-pipeline-premerge
@@ -23,6 +23,16 @@ set -o pipefail
 # Environment variables script works with:
 # List of files affected by this commit
 : ${MODIFIED_FILES:=$(git diff --name-only HEAD~1)}
+# Fetch origin/main to have an up to date merge base for main...HEAD diff.
+git fetch origin main:main
+echo "files modified HEAD~1" >&2
+git --no-pager diff --name-only HEAD~1 >&2
+echo "files modified main...HEAD" >&2
+git --no-pager diff --name-only main...HEAD | head -n 10 >&2
+merge_base=$(git merge-base main HEAD)
+echo "merge base with main $merge_base" >&2
+echo "git log" >&2
+git --no-pager log --oneline --abbrev-commit -n 5 >&2
 # Filter rules for generic windows tests
 : ${WINDOWS_AGENTS:='{"queue": "windows"}'}
 # Filter rules for generic linux tests