strbuf.c: add `strbuf_insertf()` and `strbuf_vinsertf()`

Implement `strbuf_insertf()` and `strbuf_vinsertf()` to
insert data using a printf format string.

Original-idea-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Paul-Sebastian Ungureanu <ungureanupaulsebastian@gmail.com>
Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.c

@@ -249,6 +249,42 @@ void strbuf_insert(struct strbuf *sb, size_t pos, const void *data, size_t len)
 	strbuf_splice(sb, pos, 0, data, len);
 }
 
+void strbuf_vinsertf(struct strbuf *sb, size_t pos, const char *fmt, va_list ap)
+{
+	int len, len2;
+	char save;
+	va_list cp;
+
+	if (pos > sb->len)
+		die("`pos' is too far after the end of the buffer");
+	va_copy(cp, ap);
+	len = vsnprintf(sb->buf + sb->len, 0, fmt, cp);
+	va_end(cp);
+	if (len < 0)
+		BUG("your vsnprintf is broken (returned %d)", len);
+	if (!len)
+		return; /* nothing to do */
+	if (unsigned_add_overflows(sb->len, len))
+		die("you want to use way too much memory");
+	strbuf_grow(sb, len);
+	memmove(sb->buf + pos + len, sb->buf + pos, sb->len - pos);
+	/* vsnprintf() will append a NUL, overwriting one of our characters */
+	save = sb->buf[pos + len];
+	len2 = vsnprintf(sb->buf + pos, len + 1, fmt, ap);
+	sb->buf[pos + len] = save;
+	if (len2 != len)
+		BUG("your vsnprintf is broken (returns inconsistent lengths)");
+	strbuf_setlen(sb, sb->len + len);
+}
+
+void strbuf_insertf(struct strbuf *sb, size_t pos, const char *fmt, ...)
+{
+	va_list ap;
+	va_start(ap, fmt);
+	strbuf_vinsertf(sb, pos, fmt, ap);
+	va_end(ap);
+}
+
 void strbuf_remove(struct strbuf *sb, size_t pos, size_t len)
 {
 	strbuf_splice(sb, pos, len, "", 0);

strbuf.h

@@ -244,6 +244,15 @@ void strbuf_addchars(struct strbuf *sb, int c, size_t n);
  */
 void strbuf_insert(struct strbuf *sb, size_t pos, const void *, size_t);
 
+/**
+ * Insert data to the given position of the buffer giving a printf format
+ * string. The contents will be shifted, not overwritten.
+ */
+void strbuf_vinsertf(struct strbuf *sb, size_t pos, const char *fmt,
+		     va_list ap);
+
+void strbuf_insertf(struct strbuf *sb, size_t pos, const char *fmt, ...);
+
 /**
  * Remove given amount of data from a given position of the buffer.
  */