write_file: add pointer+len variant

There are many callsites which could use write_file, but for
which it is a little awkward because they have a strbuf or
other pointer/len combo. Specifically:

 1. write_file() takes a format string, so we have to use
    "%s" or "%.*s", which are ugly.

 2. Using any form of "%s" does not handle embedded NULs in
    the output. That probably doesn't matter for our
    call-sites, but it's nicer not to have to worry.

 3. It's less efficient; we format into another strbuf
    just to do the write. That's probably not measurably
    slow for our uses, but it's simply inelegant.

We can fix this by providing a helper to write out the
formatted buffer, and just calling it from write_file().

Note that we don't do the usual "complete with a newline"
that write_file does. If the caller has their own buffer,
there's a reasonable chance they're doing something more
complicated than a single line, and they can call
strbuf_complete_line() themselves.

We could go even further and add strbuf_write_file(), but it
doesn't save much:

  -  write_file_buf(path, sb.buf, sb.len);
  +  strbuf_write_file(&sb, path);

It would also be somewhat asymmetric with strbuf_read_file,
which actually returns errors rather than dying (and the
error handling is most of the benefit of write_file() in the
first place).

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

cache.h

@@ -1734,6 +1734,12 @@ static inline ssize_t write_str_in_full(int fd, const char *str)
 	return write_in_full(fd, str, strlen(str));
 }
 
+/**
+ * Open (and truncate) the file at path, write the contents of buf to it,
+ * and close it. Dies if any errors are encountered.
+ */
+extern void write_file_buf(const char *path, const char *buf, size_t len);
+
 extern void write_file(const char *path, const char *fmt, ...);
 
 /* pager.c */

wrapper.c

@@ -640,22 +640,28 @@ int xsnprintf(char *dst, size_t max, const char *fmt, ...)
 	return len;
 }
 
+void write_file_buf(const char *path, const char *buf, size_t len)
+{
+	int fd = xopen(path, O_WRONLY | O_CREAT | O_TRUNC, 0666);
+	if (write_in_full(fd, buf, len) != len)
+		die_errno(_("could not write to %s"), path);
+	if (close(fd))
+		die_errno(_("could not close %s"), path);
+}
+
 void write_file(const char *path, const char *fmt, ...)
 {
 	va_list params;
 	struct strbuf sb = STRBUF_INIT;
-	int fd = xopen(path, O_WRONLY | O_CREAT | O_TRUNC, 0666);
 
 	va_start(params, fmt);
 	strbuf_vaddf(&sb, fmt, params);
 	va_end(params);
 
 	strbuf_complete_line(&sb);
-	if (write_in_full(fd, sb.buf, sb.len) != sb.len)
-		die_errno(_("could not write to %s"), path);
+
+	write_file_buf(path, sb.buf, sb.len);
 	strbuf_release(&sb);
-	if (close(fd))
-		die_errno(_("could not close %s"), path);
 }
 
 void sleep_millisec(int millisec)