parse-options: only insert newline in help text if needed

parse-options: only insert newline in help text if needed

Currently, when parse_options() produces a help message it always emits
a blank line after the usage text to separate it from the options text.
If the option spec does not define any switches, or only defines hidden
switches that will not be displayed, then the help text will end up with
two trailing blank lines instead of one.  Let's defer emitting the blank
line between the usage text and the options text until it is clear that
the options section will not be empty.

Fixes t1502.5, t1502.6.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>