[llvm] [ci] diff with main merge-base (PR #69308)

[Mikhail Goncharov via llvm-commits]

https://github.com/metaflow created https://github.com/llvm/llvm-project/pull/69308

Basically a reland of ec9d80ec43f5761a34c4a785c67d9e7d21ec8bda but now with fetching of main before that to get a correct merge base.

>From 4499e0fa2af4ece6a5a401c69224aed65b578c77 Mon Sep 17 00:00:00 2001
From: Mikhail Goncharov <goncharov.mikhail at gmail.com>
Date: Tue, 17 Oct 2023 11:18:54 +0200
Subject: [PATCH] [ci] diff with main merge-base

Basically a reland of ec9d80ec43f5761a34c4a785c67d9e7d21ec8bda but now
with fetching of main before that to get a correct merge base.
---
 .ci/generate-buildkite-pipeline-premerge | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/.ci/generate-buildkite-pipeline-premerge b/.ci/generate-buildkite-pipeline-premerge
index 1028c08e20fcd0a..9c6f5aefd6de005 100755
--- a/.ci/generate-buildkite-pipeline-premerge
+++ b/.ci/generate-buildkite-pipeline-premerge
@@ -21,18 +21,11 @@ set -eu
 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
+# List of files affected by this commit
+: ${MODIFIED_FILES:=$(git diff --name-only main...HEAD)}
 # Filter rules for generic windows tests
 : ${WINDOWS_AGENTS:='{"queue": "windows"}'}
 # Filter rules for generic linux tests