add strbuf_adddup()

Add a new function, strbuf_adddup(), that appends a duplicate of a
part of a struct strbuf to end of the latter.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.c

@@ -106,6 +106,13 @@ void strbuf_add(struct strbuf *sb, const void *data, size_t len)
 	strbuf_setlen(sb, sb->len + len);
 }
 
+void strbuf_adddup(struct strbuf *sb, size_t pos, size_t len)
+{
+	strbuf_grow(sb, len);
+	memcpy(sb->buf + sb->len, sb->buf + pos, len);
+	strbuf_setlen(sb, sb->len + len);
+}
+
 void strbuf_addf(struct strbuf *sb, const char *fmt, ...)
 {
 	int len;

strbuf.h

@@ -101,6 +101,7 @@ static inline void strbuf_addstr(struct strbuf *sb, const char *s) {
 static inline void strbuf_addbuf(struct strbuf *sb, struct strbuf *sb2) {
 	strbuf_add(sb, sb2->buf, sb2->len);
 }
+extern void strbuf_adddup(struct strbuf *sb, size_t pos, size_t len);
 
 typedef void (*expand_fn_t) (struct strbuf *sb, const char *placeholder, void *context);
 extern void strbuf_expand(struct strbuf *sb, const char *format, const char **placeholders, expand_fn_t fn, void *context);