Sometimes .git is a plain file; maybe others will follow.
This CL matches CL 21430, made in x/tools/go/vcs.

The change in the Swift test case makes the test case
pass by changing the test to match current behavior,
which I assume is better than the reverse.
(The test only runs locally and without -short, so the
builders are not seeing this particular failure.)

For #10322.

Change-Id: Iccd08819a01c5609a2880b9d8a99af936e20faff
Reviewed-on: https://go-review.googlesource.com/30948
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>