run_processes_parallel: treat output of children as byte array

We do not want the output to be interrupted by a NUL byte, so we
cannot use raw fputs. Introduce strbuf_write to avoid having long
arguments in run-command.c.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -386,6 +386,12 @@ extern ssize_t strbuf_read_file(struct strbuf *sb, const char *path, size_t hint
  */
 extern int strbuf_readlink(struct strbuf *sb, const char *path, size_t hint);
 
+/**
+ * Write the whole content of the strbuf to the stream not stopping at
+ * NUL bytes.
+ */
+extern ssize_t strbuf_write(struct strbuf *sb, FILE *stream);
+
 /**
  * Read a line from a FILE *, overwriting the existing contents
  * of the strbuf. The second argument specifies the line