On Windows, Rel emits error messages of the form `Rel: can't make
\windows relative to \windows`. Rather than emitting paths after
stripping volume names, emit the original paths so as to make those of
the form `Rel: can't make d:\windows relative to c:\windows`.  Fixed a
test that expected the error message to emit clean path instead of the
original.

Fixes #13259

Change-Id: I3a9bd5b137205f22794ec8046b4e917ee48cf750
Reviewed-on: https://go-review.googlesource.com/16858
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>