t7600: clean up style

t7600: clean up style

Clean up the 'merge --squash c3 with c7' test by removing some
unnecessary braces and removing a pipe.

Also, generally cleanup style by unindenting a here-doc, removing stray
spaces after a redirection operator and allowing sed to open its own
input instead of redirecting input from the shell.

Helped-by: SZEDER Gábor <redacted>
Helped-by: Eric Sunshine <redacted>
Helped-by: Junio C Hamano <redacted>
Signed-off-by: Denton Liu <redacted>
Signed-off-by: Junio C Hamano <redacted>