strbuf: add strbuf_addbuf_percentquote

This is handy for creating strings which will be fed to printf() or
strbuf_expand().

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

Documentation/technical/api-strbuf.txt

@@ -218,6 +218,13 @@ which can be used by the programmer of the callback as she sees fit.
 	placeholder and replacement string.  The array needs to be
 	terminated by an entry with placeholder set to NULL.
 
+`strbuf_addbuf_percentquote`::
+
+	Append the contents of one strbuf to another, quoting any
+	percent signs ("%") into double-percents ("%%") in the
+	destination. This is useful for literal data to be fed to either
+	strbuf_expand or to the *printf family of functions.
+
 `strbuf_addf`::
 
 	Add a formatted string to the buffer.

strbuf.c

@@ -257,6 +257,17 @@ size_t strbuf_expand_dict_cb(struct strbuf *sb, const char *placeholder,
 	return 0;
 }
 
+void strbuf_addbuf_percentquote(struct strbuf *dst, const struct strbuf *src)
+{
+	int i, len = src->len;
+
+	for (i = 0; i < len; i++) {
+		if (src->buf[i] == '%')
+			strbuf_addch(dst, '%');
+		strbuf_addch(dst, src->buf[i]);
+	}
+}
+
 size_t strbuf_fread(struct strbuf *sb, size_t size, FILE *f)
 {
 	size_t res;

strbuf.h

@@ -116,6 +116,7 @@ struct strbuf_expand_dict_entry {
 	const char *value;
 };
 extern size_t strbuf_expand_dict_cb(struct strbuf *sb, const char *placeholder, void *context);
+extern void strbuf_addbuf_percentquote(struct strbuf *dst, const struct strbuf *src);
 
 __attribute__((format (printf,2,3)))
 extern void strbuf_addf(struct strbuf *sb, const char *fmt, ...);