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>