factor out strbuf_expand_bad_format()

factor out strbuf_expand_bad_format()

Extract a function for reporting placeholders that are not enclosed in a
parenthesis or are unknown.  This reduces the number of strings to
translate and improves consistency across commands.  Call it at the end
of the if/else chain, after exhausting all accepted possibilities.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>