go/vcs: accept plain file for .vcs (instead of directory)

Sometimes .git is a plain file; maybe others will follow.

Fixes golang/go#10322.

Change-Id: Id57424998c207080c3ed5826b1e5e091fa26aad5
Reviewed-on: https://go-review.googlesource.com/21430
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
2 files changed