pkt-line: add stdio packet write functions

This adds three new functions to pkt-line.c: packet_fwrite,
packet_fwrite_fmt and packet_fflush. Besides writing a pktline flush
packet, packet_fflush also flushes the stdio buffer of the stream.

Helped-by: Patrick Steinhardt <ps@pks.im>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Jacob Vosmaer <jacob@gitlab.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pkt-line.h

@@ -35,6 +35,17 @@ int packet_write_fmt_gently(int fd, const char *fmt, ...) __attribute__((format
 int write_packetized_from_fd_no_flush(int fd_in, int fd_out);
 int write_packetized_from_buf_no_flush(const char *src_in, size_t len, int fd_out);
 
+/*
+ * Stdio versions of packet_write functions. When mixing these with fd
+ * based functions, take care to call fflush(3) before doing fd writes or
+ * closing the fd.
+ */
+void packet_fwrite(FILE *f, const char *buf, size_t size);
+void packet_fwrite_fmt(FILE *f, const char *fmt, ...) __attribute__((format (printf, 2, 3)));
+
+/* packet_fflush writes a flush packet and flushes the stdio buffer of f */
+void packet_fflush(FILE *f);
+
 /*
  * Read a packetized line into the buffer, which must be at least size bytes
  * long. The return value specifies the number of bytes read into the buffer.