[git] Git merge reports "Already up-to-date" though there is a difference

A merge is always between the current HEAD and one or more commits (usually, branch head or tag),
and the index file must match the tree of HEAD commit (i.e. the contents of the last commit) when it starts out.
In other words, git diff --cached HEAD must report no changes.

The merged commit is already contained in HEAD. This is the simplest case, called "Already up-to-date."

That should mean the commits in test are already merged in master, but since other commits are done on master, git diff test would still give some differences.