CL 118095 added gitrepo tests. These tests are failing on Plan 9
since they expect a full-featured git command, while the git tool
has been emulated as a simple rc script on Plan 9.

Fixes #25938.

Change-Id: I262a89d0ce83168c550d9af3e832ed3a1e3c43f6
Reviewed-on: https://go-review.googlesource.com/122455
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>