fix: error overriding the base sha (#933)

diff-sha.sh

@@ -215,21 +215,16 @@ else
       PREVIOUS_SHA=$GITHUB_EVENT_PULL_REQUEST_BASE_SHA && exit_status=$? || exit_status=$?
     fi
 
+    if ! git diff --name-only --ignore-submodules=all "$PREVIOUS_SHA$DIFF$CURRENT_SHA" 1>/dev/null 2>&1; then
+        PREVIOUS_SHA=$(git rev-parse "$TARGET_BRANCH") && exit_status=$? || exit_status=$?
+        DIFF=".."
+    fi
+
     echo "::debug::Previous SHA: $PREVIOUS_SHA"
   else
     PREVIOUS_SHA=$INPUT_BASE_SHA && exit_status=$? || exit_status=$?
   fi
 
-  if ! git diff --name-only --ignore-submodules=all "$PREVIOUS_SHA$DIFF$CURRENT_SHA" 1>/dev/null 2>&1; then
-    PREVIOUS_SHA=$(git rev-parse "$TARGET_BRANCH") && exit_status=$? || exit_status=$?
-    DIFF=".."
-
-    if ! git diff --name-only --ignore-submodules=all "$PREVIOUS_SHA$DIFF$CURRENT_SHA" 1>/dev/null 2>&1; then
-      echo "::error::Unable find a diff between $PREVIOUS_SHA and $CURRENT_SHA"
-      exit 1
-    fi
-  fi
-
   echo "::debug::Target branch: $TARGET_BRANCH"
   echo "::debug::Current branch: $CURRENT_BRANCH"
 
@@ -241,6 +236,11 @@ else
     echo "::error::Please verify that the previous sha is valid, and increase the fetch_depth to a number higher than $INPUT_FETCH_DEPTH."
     exit 1
   fi
+
+  if ! git diff --name-only --ignore-submodules=all "$PREVIOUS_SHA$DIFF$CURRENT_SHA" 1>/dev/null 2>&1; then
+    echo "::error::Unable find a diff between $PREVIOUS_SHA and $CURRENT_SHA"
+    exit 1
+  fi
 fi
 
 if [[ "$PREVIOUS_SHA" == "$CURRENT_SHA" && "$INITIAL_COMMIT" == "false" ]]; then