fix pipe usage for linux This uses the correct read and write ends of the fds returned by pipe(2) on linux. It also moves away from creating FILE* streams with fdopen and reading the stream with buf_load (which performs a fstat, and breaks due to a zero length file size on linux) by instead reading from the fd itself. Reported by abieber@, with assistance from stsp@ ok stsp@

make merge_file() accept FILEs instead of paths for orig and deriv inputs

kn@ and stsp@ suggested changes

remove worklist code; it was only used to unlink files which we already unlink

more mechanical conversions to struct got_error: buf_load() and buf_alloc()

get rid of xmalloc and err functions in buf.c

add unmodified buf.c, buf.h, and diff3.c from OpenRCS