cmd/go: enable file shortening for lines starting with \t

Compiler errors now (as of 1.18) might start with a tab character,
for errors which take multiple lines to report. e.g.:

/Users/khr/gowork/tmp1.go:3:15: x redeclared in this block
	/Users/khr/gowork/tmp1.go:3:8: other declaration of x

This CL makes error lines starting with a tab character
eligible for replacing absolute paths with relative ones.

Fixes #51177

Change-Id: Ic9e9c610a1aa1e21e9f19e6a9bd05c73b5a14e4a
Reviewed-on: https://go-review.googlesource.com/c/go/+/385755


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Trust: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>