nightly_revert_checker: update branch name

Android updated this branch name, so we have to reflect that here.

BUG=b:182277561
TEST=None

Change-Id: Ia709901bf8544eb751bc4c736f9e351640c52329
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2746504
Reviewed-by: Pirama Arumuga Nainar <pirama@google.com>
Reviewed-by: George Burgess <gbiv@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
diff --git a/llvm_tools/nightly_revert_checker.py b/llvm_tools/nightly_revert_checker.py
index 21dcf94..d8566c3 100755
--- a/llvm_tools/nightly_revert_checker.py
+++ b/llvm_tools/nightly_revert_checker.py
@@ -41,7 +41,7 @@
 
   def get_llvm_merge_base(branch: str) -> str:
     return subprocess.check_output(
-        ['git', 'merge-base', branch, 'aosp/upstream-master'],
+        ['git', 'merge-base', branch, 'aosp/upstream-main'],
         cwd=llvm_project,
         encoding='utf-8',
     ).strip()