Only use --unshallow for shallow repositories

Author: jsoref

merge

@@ -20,7 +20,10 @@ if [ -z "$GIT_AUTHOR_EMAIL" ]; then
 fi
 export GIT_COMMITTER_NAME=GitHub
 export GIT_COMMITTER_EMAIL=noreply@github.com
-git fetch --unshallow origin "$INPUT_BASE_REF" ||
+if [ "$(git rev-parse --is-shallow-repository)" = 'true' ]; then
+    UNSHALLOW=--unshallow
+fi
+git fetch $UNSHALLOW origin "$INPUT_BASE_REF" ||
     report_failure "Can't get history for base_ref ($INPUT_BASE_REF). Please contact support for $GITHUB_ACTION_REPOSITORY"
 GITHUB_BASE_SHA=$(git rev-parse FETCH_HEAD)