cmd/go: skip the 'git' part of get_insecure_redirect in short mode

cmd/go: skip the 'git' part of get_insecure_redirect in short mode

Invoking 'git' adds about 200ms to this test on a fast machine,
probably more on a slow one. (As a general habit we skip the
'git' tests uniformly in short mode.)

For #52545.

Change-Id: Iea6d86a8c9c8b0f1fe51888faf7f5fe7dd8f1eb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/488236
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>