help: don't print "\n" before single-section output

help: don't print "\n" before single-section output

Fix a formatting regression in 1b81d8cb19d (help: use command-list.txt
for the source of guides, 2018-05-20). Adjust the output of "git help
--guides" and any other future single-section commands so that a
newline isn't inserted before the only section being printed.

This changes the output from:

    $ git help --guides

    The Git concept guides are:
    [...]

To:

    $ git help --guides
    The Git concept guides are:
    [...]

That we started printing an extra "\n" in 1b81d8cb19d wasn't intended,
but an emergent effect of moving all of the printing of "git help"
output to code that was ready to handle printing N sections.

With 1b81d8cb19d we started using the "print_cmd_by_category()"
function added earlier in the same series, or in cfb22a02ab5 (help:
use command-list.h for common command list, 2018-05-10).

Fixing this formatting nit is easy enough. Let's have all of the
output that would like to be "\n"-separated from other lines emit its
own "\n". We then adjust "print_cmd_by_category()" to only print a
"\n" to delimit the sections it's printing out.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>