git_connect: let user override virtual-host we send to daemon

git_connect: let user override virtual-host we send to daemon

When we connect to a git-daemon at a given host and port, we
actually send the string "localhost:9418" to the other side,
which allows it to do virtual-hosting lookups. For testing
and debugging, we'd like to be able to send arbitrary
strings, rather than the hostname we actually connected to.

Using "insteadOf" config does not work for this purpose, as
the hostname determination happens at a very low level,
right before we feed the hostname to our lookup routines.
You could use /etc/hosts or similar to get around this, but
we cannot do that portably from our test suite.

Instead, this patch provides an environment variable that
can be used to send an arbitrary string.

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