[Github] Only look at previous commit for MacOS Premerge (llvm#164483)

Otherwise we're looking for a commit that does not exist given the fetch
depth.

Fixes llvm#164430.

(cherry picked from commit 288ef04)

Author: boomanaiden154

.github/workflows/premerge.yaml

@@ -149,7 +149,7 @@ jobs:
         uses: llvm/actions/install-ninja@main
       - name: Build and Test
         run: |
-          source <(git diff --name-only HEAD~2..HEAD | python3 .ci/compute_projects.py)
+          source <(git diff --name-only HEAD~1...HEAD | python3 .ci/compute_projects.py)
 
           if [[ "${projects_to_build}" == "" ]]; then
             echo "No projects to build"