http-backend: respect CONTENT_LENGTH as specified by rfc3875

http-backend: respect CONTENT_LENGTH as specified by rfc3875

http-backend reads whole input until EOF. However, the RFC 3875 specifies
that a script must read only as many bytes as specified by CONTENT_LENGTH
environment variable. Web server may exercise the specification by not closing
the script's standard input after writing content. In that case http-backend
would hang waiting for the input. The issue is known to happen with
IIS/Windows, for example.

Make http-backend read only CONTENT_LENGTH bytes, if it's defined, rather than
the whole input until EOF. If the variable is not defined, keep older behavior
of reading until EOF because it is used to support chunked transfer-encoding.

This commit only fixes buffered input, whcih reads whole body before
processign it. Non-buffered input is going to be fixed in subsequent commit.

Signed-off-by: Florian Manschwetus <manschwetus@cs-software-gmbh.de>
[mk: fixed trivial build failures and polished style issues]
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Max Kirillov <max@max630.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>