Use GITHUB_SHA for finding pr base commit (#6)

Per documentation:

GITHUB_SHA is Last merge commit on the GITHUB_REF branch
and GITHUB_REF is PR merge branch refs/pull/:prNumber/merge

Therefore, we could use GITHUB_SHA~commitNumber for pr base commit.
diff --git a/diff-only/entrypoint.sh b/diff-only/entrypoint.sh
index 2084b30..2a5a6b2 100755
--- a/diff-only/entrypoint.sh
+++ b/diff-only/entrypoint.sh
@@ -7,9 +7,8 @@
 echo "COMMITS: ${COMMITS}"
 
 git --version
-git rev-parse --abbrev-ref HEAD
 
-CHANGED_DIRS=$(git diff --dirstat=files,0 HEAD~${COMMITS} | awk '{ print $2 }')
+CHANGED_DIRS=$(git diff --dirstat=files,0 ${GITHUB_SHA}~${COMMITS} | awk '{ print $2 }')
 echo "CHANGED_DIRS are : ${CHANGED_DIRS}"
 
 found_changed_dir_not_in_target_dirs="no"
@@ -35,4 +34,4 @@
 else
     echo "Changes ${CHANGED_DIRS} only in $*, setting 'changed_only' to 'yes'"
     echo ::set-output name=changed_only::yes
-fi
\ No newline at end of file
+fi