Add missing changelog for #13016 (#13812)

* Add missing changelog

* Fix

changelog_unreleased/cli/13016.md

@@ -0,0 +1,29 @@
+#### Fix `--cache` set only if with `--write` or `--no-different` (#13016 by @Milly)
+
+Do not save formatted cache if without `--write` option and the file is modifies by prettier.
+
+Before fixed, always save formatted cache if with `--cache` option.
+Therefore, unformatted files will never be formatted.
+
+**2.7:**
+
+```
+> prettier --cache foo.js
+# Show formatted contents of `foo.js`.
+# Then the cache is created and `foo.js` is flagged as already formatted.
+
+> prettier --cache --write foo.js
+foo.js 2ms (cached)
+# "... (cached)" means that the file is already formatted and do nothing this time.
+```
+
+**2.8:**
+
+```
+> prettier --cache foo.js
+# Show formatted contents of `foo.js`.
+
+> prettier --cache --write foo.js
+foo.js 2ms
+# `foo.js` has
+```