The http-backend (used for smart-http transport) used to slurp the
whole input until EOF, without paying attention to CONTENT_LENGTH
that is supplied in the environment and instead expecting the Web
server to close the input stream.  This has been fixed.

* mk/http-backend-content-length:
  t5562: avoid non-portable "export FOO=bar" construct
  http-backend: respect CONTENT_LENGTH for receive-pack
  http-backend: respect CONTENT_LENGTH as specified by rfc3875
  http-backend: cleanup writing to child process