revert: actually check for a dirty index

revert: actually check for a dirty index

The previous code mistakenly used wt_status_prepare to check whether the
index had anything commitable in it; however, that function is just an
init function, and will never report a dirty index.

The correct way with wt_status_* would be to call wt_status_print with the
output pointing to /dev/null or similar. However, that does extra work by
both examining the working tree and spewing status information to nowhere.

Instead, let's just implement the useful subset of wt_status_print as an
"is_index_dirty" function.

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