t5537: use test_write_lines and indented heredocs for readability

t5537: use test_write_lines and indented heredocs for readability

A number of spots in t5537 use the non-indented heredoc '<<EOF' when
they would benefit from instead using '<<-EOF' or simply
test_write_lines.

In preparation for adding new tests in a good style and being consistent
with the surrounding code, update the existing tests to improve their
readability.

Suggested-by: Junio C Hamano <redacted>
Signed-off-by: Taylor Blau <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>