t3900: do not reference numbered arguments from the test script

The call to test_expect_success is nested inside a function, whose arguments the test code wants to access. But it is not specified that any unexpanded $1, $2, $3, etc in the test code will access the surrounding function's arguments. Rather, they will access the arguments of the function that happens to eval the test code. In this case, the reference is intended to supply '-m message' to a call of 'git commit --squash'. Remove it because -m is optional and the test case does not check for it. There are tests in t7500 that check combinations of --squash and -m. Signed-off-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>