GIT_TRACE write message to stderr, while run1 merge both stdout and
stderr. So function which call run1 and rely on its output will failed
to parse the result when run1 success.

By using cmd.Output(), we ensure only cmd standard out is returned.

Fixes #19682

Change-Id: I7002df17fe68aea1860ddc7382c68cc23548bd90
Reviewed-on: https://go-review.googlesource.com/126735Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>