strbuf_add_commented_lines(): avoid SP-HT sequence in commented lines

strbuf_add_commented_lines(): avoid SP-HT sequence in commented lines

The strbuf_add_commented_lines() function passes a pair of prefixes,
one to be used for a non-empty line, and the other for an empty
line, to underlying add_lines().  The former is set to a comment
char followed by a SP, while the latter is set to just the comment
char.  This is designed to give a SP after the comment character,
e.g. "# <user text>\n", on a line with some text, and to avoid
emitting an unsightly "# \n" for an empty line.

Teach this machinery to also use the latter space-less prefix when
the payload line begins with a tab, to show e.g. "#\t<user text>\n";
otherwise we will end up showing "# \t<user text>\n" which is
similarly unsightly.

Signed-off-by: Junio C Hamano <gitster@pobox.com>