Add new function strbuf_add_xml_quoted()

Substantially the same code is present in http-push.c and imap-send.c,
so make a library function out of it.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.c

@@ -428,6 +428,32 @@ void strbuf_add_lines(struct strbuf *out, const char *prefix,
 	strbuf_complete_line(out);
 }
 
+void strbuf_addstr_xml_quoted(struct strbuf *buf, const char *s)
+{
+	while (*s) {
+		size_t len = strcspn(s, "\"<>&");
+		strbuf_add(buf, s, len);
+		s += len;
+		switch (*s) {
+		case '"':
+			strbuf_addstr(buf, "&quot;");
+			break;
+		case '<':
+			strbuf_addstr(buf, "&lt;");
+			break;
+		case '>':
+			strbuf_addstr(buf, "&gt;");
+			break;
+		case '&':
+			strbuf_addstr(buf, "&amp;");
+			break;
+		case 0:
+			return;
+		}
+		s++;
+	}
+}
+
 static int is_rfc3986_reserved(char ch)
 {
 	switch (ch) {

strbuf.h

@@ -102,6 +102,12 @@ extern void strbuf_vaddf(struct strbuf *sb, const char *fmt, va_list ap);
 
 extern void strbuf_add_lines(struct strbuf *sb, const char *prefix, const char *buf, size_t size);
 
+/*
+ * Append s to sb, with the characters '<', '>', '&' and '"' converted
+ * into XML entities.
+ */
+extern void strbuf_addstr_xml_quoted(struct strbuf *sb, const char *s);
+
 static inline void strbuf_complete_line(struct strbuf *sb)
 {
 	if (sb->len && sb->buf[sb->len - 1] != '\n')