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>

strbuf.h

@@ -337,6 +337,11 @@ size_t strbuf_expand_literal(struct strbuf *sb, const char *placeholder);
  */
 int strbuf_expand_step(struct strbuf *sb, const char **formatp);
 
+/**
+ * Used with `strbuf_expand_step` to report unknown placeholders.
+ */
+void strbuf_expand_bad_format(const char *format, const char *command);
+
 /**
  * Append the contents of one strbuf to another, quoting any
  * percent signs ("%") into double-percents ("%%") in the