t0056: "git -C" test updates

Instead of repeating the text to record as the commit log message
and string we expect to see in "log" output, use the same variable
to avoid them going out of sync.

Use different names for test files in different directories to
improve our chance to catch future breakages that makes "-C <dir>"
go to a place that is different from what was specified.

Signed-off-by: Nazri Ramliy <ayiehere@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>