t/lib-read-tree-m-3way: modernize style

t/lib-read-tree-m-3way: modernize style

Many invocations of the test_expect_success command in this
file are written in old style where the command, an optional
prerequisite, and the test title are written on separate
lines, and the executable script string begins on its own
line, and these lines are pasted together with backslashes
as necessary.

An invocation of the test_expect_success command in modern
test scripts however writes the prerequisite and the title
on the same line as the test_expect_success command itself,
and ends the line with a single quote that begins the
executable script string.

Update the style for uniformity.

Signed-off-by: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>