strbuf: simplify comment-handling in add_lines() helper

strbuf: simplify comment-handling in add_lines() helper

In strbuf_add_commented_lines(), we prepare two strings with potential
prefixes: one with just the comment char, and one with an additional
space. In the add_lines() helper, we use the one without the extra space
for blank lines or lines starting with a tab.

While passing in two separate prefixes to the helper is very flexible,
it's more flexibility than we actually use (or are likely to use, since
the rules inside add_lines() only make sense if "prefix2" is a variant
of "prefix1" without the extra space). And setting up the two strings
makes refactoring in strbuf_add_commented_lines() awkward.

Instead, let's pass in a single string, and just let add_lines() add the
extra space to the result as appropriate.

We do still need to pass in a flag to trigger this behavior. The helper
is shared by strbuf_add_lines(), which passes in a NULL "prefix2" to
inhibit this extra handling.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>