t6044: add more testcases with staged changes before a merge is invoked

t6044: add more testcases with staged changes before a merge is invoked

According to Documentation/git-merge.txt,

    ...[merge will] abort if there are any changes registered in the index
    relative to the `HEAD` commit.  (One exception is when the changed
    index entries are in the state that would result from the merge
    already.)

Add some tests showing that this exception, while it does accurately state
what would be a safe condition under which we could allow the merge to
proceed, is not what is actually implemented.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>