daemon: recognize hidden request arguments

daemon: recognize hidden request arguments

A normal request to git-daemon is structured as
"command path/to/repo\0host=..\0" and due to a bug introduced in
49ba83fb6 (Add virtualization support to git-daemon, 2006-09-19) we
aren't able to place any extra arguments (separated by NULs) besides the
host otherwise the parsing of those arguments would enter an infinite
loop.  This bug was fixed in 73bb33a94 (daemon: Strictly parse the
"extra arg" part of the command, 2009-06-04) but a check was put in
place to disallow extra arguments so that new clients wouldn't trigger
this bug in older servers.

In order to get around this limitation teach git-daemon to recognize
additional request arguments hidden behind a second NUL byte.  Requests
can then be structured like:
"command path/to/repo\0host=..\0\0version=1\0key=value\0".  git-daemon
can then parse out the extra arguments and set 'GIT_PROTOCOL'
accordingly.

By placing these extra arguments behind a second NUL byte we can skirt
around both the infinite loop bug in 49ba83fb6 (Add virtualization
support to git-daemon, 2006-09-19) as well as the explicit disallowing
of extra arguments introduced in 73bb33a94 (daemon: Strictly parse the
"extra arg" part of the command, 2009-06-04) because both of these
versions of git-daemon check for a single NUL byte after the host
argument before terminating the argument parsing.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>