This makes custom import path checks work even when the
custom import metadata directs checking out a subtree
of the subversion repository.

(Git and Mercurial allow no such thing, so they are unaffected.)

Fixes #20731.

Change-Id: I635f3a2037d69a87c6dac7b08b0a0d8266abd250
Reviewed-on: https://go-review.googlesource.com/46417
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>