cmd/goimports: permit complete filename for -srcdir
cmd/goimports: permit complete filename for -srcdir

Updates dominikh/go-mode.el#146
Updates golang/go#7463 (for https://golang.org/cl/23444)

Change-Id: Ieb769329531050b803528acce0c50d02786533b6
Reviewed-on: https://go-review.googlesource.com/25140
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

golang/tools,syreclabs/go-tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,golang/tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools