lintcmd: make binary output OS agnostic

We were storing absolute paths and full token.Position in the binary
output. This works fine when merging multiple runs from the same OS, but
not so when dealing with multiple OSs, which have different path
separators and different absolute paths. We also have to zero out
token.Position.Offset to handle different newlines on different OSs.

(cherry picked from commit 87cceb1)

lintcmd/cmd.go

@@ -11,6 +11,7 @@ import (
 	"io"
 	"log"
 	"os"
+	"path/filepath"
 	"reflect"
 	"runtime"
 	"runtime/pprof"
@@ -436,7 +437,46 @@ func (cmd *Command) Run() {
 				fmt.Fprintln(os.Stderr, "warning:", w)
 			}
 
+			cwd, err := os.Getwd()
+			if err != nil {
+				cwd = ""
+			}
+			relPath := func(s string) string {
+				if cwd == "" {
+					return filepath.ToSlash(s)
+				}
+				out, err := filepath.Rel(cwd, s)
+				if err != nil {
+					return filepath.ToSlash(s)
+				}
+				return filepath.ToSlash(out)
+			}
+
 			if cmd.flags.formatter == "binary" {
+				for i, s := range res.CheckedFiles {
+					res.CheckedFiles[i] = relPath(s)
+				}
+				for i := range res.Diagnostics {
+					// We turn all paths into relative, /-separated paths. This is to make -merge work correctly when
+					// merging runs from different OSs, with different absolute paths.
+					//
+					// We zero out Offset, because checkouts of code on different OSs may have different kinds of
+					// newlines and thus different offsets. We don't ever make use of the Offset, anyway. Line and
+					// column numbers are precomputed.
+
+					d := &res.Diagnostics[i]
+					d.Position.Filename = relPath(d.Position.Filename)
+					d.Position.Offset = 0
+					d.End.Filename = relPath(d.End.Filename)
+					d.End.Offset = 0
+					for j := range d.Related {
+						r := &d.Related[j]
+						r.Position.Filename = relPath(r.Position.Filename)
+						r.Position.Offset = 0
+						r.End.Filename = relPath(r.End.Filename)
+						r.End.Offset = 0
+					}
+				}
 				err := gob.NewEncoder(os.Stdout).Encode(res)
 				if err != nil {
 					fmt.Fprintf(os.Stderr, "failed writing output: %s\n", err)