strbuf: add strbuf_reencode helper

This is a convenience wrapper around `reencode_string_len`
and `strbuf_attach`.

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

Documentation/technical/api-strbuf.txt

@@ -125,6 +125,11 @@ Functions
 
 	Strip whitespace from the end of a string.
 
+`strbuf_reencode`::
+
+	Replace the contents of the strbuf with a reencoded form.  Returns -1
+	on error, 0 on success.
+
 `strbuf_cmp`::
 
 	Compare two buffers. Returns an integer less than, equal to, or greater

strbuf.c

@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "refs.h"
+#include "utf8.h"
 
 int starts_with(const char *str, const char *prefix)
 {
@@ -106,6 +107,22 @@ void strbuf_ltrim(struct strbuf *sb)
 	sb->buf[sb->len] = '\0';
 }
 
+int strbuf_reencode(struct strbuf *sb, const char *from, const char *to)
+{
+	char *out;
+	int len;
+
+	if (same_encoding(from, to))
+		return 0;
+
+	out = reencode_string_len(sb->buf, sb->len, to, from, &len);
+	if (!out)
+		return -1;
+
+	strbuf_attach(sb, out, len, len);
+	return 0;
+}
+
 struct strbuf **strbuf_split_buf(const char *str, size_t slen,
 				 int terminator, int max)
 {

strbuf.h

@@ -45,6 +45,7 @@ static inline void strbuf_setlen(struct strbuf *sb, size_t len)
 extern void strbuf_trim(struct strbuf *);
 extern void strbuf_rtrim(struct strbuf *);
 extern void strbuf_ltrim(struct strbuf *);
+extern int strbuf_reencode(struct strbuf *sb, const char *from, const char *to);
 extern int strbuf_cmp(const struct strbuf *, const struct strbuf *);
 
 /*