t5704: match tests to modern style

t5704: match tests to modern style

The test did not adhere to the current style on several counts:

 . empty lines around the test blocks, but within the test string
 . ': > file' or even just '> file' with an extra space
 . inconsistent indentation
 . hand-rolled commits instead of using test_commit

Fix all of them.

There's a catch to the last point: test_commit creates a tag, which the
original test did not create.  We still change it to test_commit, and
explicitly delete the tags, so as to highlight that the test relies on not
having them.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>