Brandon Williams authored Oct 16, 2017 and Junio C Hamano committed Oct 17, 2017

Teach the connection logic to tell a serve that it understands protocol
v1.  This is done in 2 different ways for the builtin transports, both
of which ultimately set 'GIT_PROTOCOL' to 'version=1' on the server.

1. git://
   A normal request to git-daemon is structured as
   "command path/to/repo\0host=..\0" and due to a bug introduced in
   49ba83fb (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 73bb33a9 (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 git-daemon was taught 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 49ba83fb (Add
   virtualization support to git-daemon, 2006-09-19) as well as the
   explicit disallowing of extra arguments introduced in 73bb33a9


   (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.

2. ssh://, file://
   Set 'GIT_PROTOCOL' environment variable with the desired protocol
   version.  With the file:// transport, 'GIT_PROTOCOL' can be set
   explicitly in the locally running git-upload-pack or git-receive-pack
   processes.  With the ssh:// transport and OpenSSH compliant ssh
   programs, 'GIT_PROTOCOL' can be sent across ssh by using '-o
   SendEnv=GIT_PROTOCOL' and having the server whitelist this
   environment variable.

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