git_connect: clarify conn->use_shell flag

git_connect: clarify conn->use_shell flag

When executing user-specified programs, we generally always
want to use a shell, for flexibility and consistency. One
big exception is executing $GIT_SSH, which for historical
reasons must not use a shell.

Once upon a time the logic in git_connect looked like:

  if (protocol == PROTO_SSH) {
	  ... setup ssh ...
  } else {
	  ... setup local connection ...
	  conn->use_shell = 1;
  }

But over time the PROTO_SSH block has grown, and the "local"
block has shrunk so that it contains only conn->use_shell;
it's easy to miss at the end of the large block.  Moreover,
PROTO_SSH now also sometimes sets use_shell, when the new
GIT_SSH_COMMAND is used.

Let's just set conn->use_shell when we're setting up the "conn"
struct, and unset it (with a comment) in the historical GIT_SSH
case.  This will make the flow easier to follow.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>