fix: override=true must not needlessly delete a tag

If the tag to be set is exactly the same as the tag that exists, do
not delete it.

When the tag is recreated although it is not necessary, it may trigger
workflows that have already been triggered and send the workflow in a
loop.

- split `get_tag` & `matched_tag` out of ensure_tag
- in case of an override, only delete the tag if it fails `matched_tag`
- trace shell lines to help debug
- add testing

testdata/forgejo-release-test.sh

@@ -45,15 +45,18 @@ test_ensure_tag() {
     # idempotent
     #
     ensure_tag
-    api GET repos/$REPO/tags/$TAG >$TMP_DIR/tag1.json
+    mv $TAG_FILE $TMP_DIR/tag1.json
+
     ensure_tag
-    api GET repos/$REPO/tags/$TAG >$TMP_DIR/tag2.json
+    mv $TAG_FILE $TMP_DIR/tag2.json
+
     diff -u $TMP_DIR/tag[12].json
     #
     # sanity check on the SHA of an existing tag
     #
     (
         SHA=12345
+        ! matched_tag
         ! ensure_tag
     )
     api DELETE repos/$REPO/tags/$TAG