write_file: add format attribute

This gives us compile-time checking of our format strings,
which is a good thing.

I had also hoped it would help with confusing write_file()
and write_file_buf(), since the former's "..." can make it
match the signature of the latter. But given that the buffer
for write_file_buf() is generally not a string literal, the
compiler won't complain unless -Wformat-nonliteral is on,
and that creates a ton of false positives elsewhere in the
code base.

While we're there, let's also give the function a docstring,
which it never had.

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

cache.h

@@ -1740,6 +1740,14 @@ static inline ssize_t write_str_in_full(int fd, const char *str)
  */
 extern void write_file_buf(const char *path, const char *buf, size_t len);
 
+/**
+ * Like write_file_buf(), but format the contents into a buffer first.
+ * Additionally, write_file() will append a newline if one is not already
+ * present, making it convenient to write text files:
+ *
+ *   write_file(path, "counter: %d", ctr);
+ */
+__attribute__((format (printf, 2, 3)))
 extern void write_file(const char *path, const char *fmt, ...);
 
 /* pager.c */