transport/internal: read only first error line
transport/internal: read only first error line

We only use the first line of error output. So we use a channel with
a single byte buffer and read the first line from stderr. Further
output is discarded, as well as any further I/O error, which might
be expected when closing the pipe (command finished).

src-d/go-git,go-git/go-git,mcuadros/go-git,alcortesm/go-git,src-d/go-git,alcortesm/go-git,mcuadros/go-git,fuchsia-mirror/third_party-github.com-src-d-go-git,fuchsia-mirror/third_party-github.com-src-d-go-git,go-git/go-git