bugreport: add tool to generate debugging info

Teach Git how to prompt the user for a good bug report: reproduction
steps, expected behavior, and actual behavior. Later, Git can learn how
to collect some diagnostic information from the repository.

If users can send us a well-written bug report which contains diagnostic
information we would otherwise need to ask the user for, we can reduce
the number of question-and-answer round trips between the reporter and
the Git contributor.

Users may also wish to send a report like this to their local "Git
expert" if they have put their repository into a state they are confused
by.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -450,6 +450,7 @@ int strbuf_readlink(struct strbuf *sb, const char *path, size_t hint);
  * NUL bytes.
  */
 ssize_t strbuf_write(struct strbuf *sb, FILE *stream);
+ssize_t strbuf_write_fd(struct strbuf *sb, int fd);
 
 /**
  * Read a line from a FILE *, overwriting the existing contents of