rev-parse test: modernize quoting and whitespace

rev-parse test: modernize quoting and whitespace

Instead of cramming everything in one line, put the test body in an
indented block after the opening test_expect_success line and quote
and put the closing quote on a line by itself.

Use single-quote instead of double-quote to quote the test body
for more useful --verbose output.

Signed-off-by: Felipe Contreras <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>