cmd/go: fix formatting of file paths under cwd

The output of go with -x flag is formatted in a manner that file paths
under current directory are modified to start with a dot (.), but when
the directory path ends with a slash (/), the formatting goes wrong.

Fixes 23982.

Author: motemen

src/cmd/go/internal/work/exec.go

@@ -1364,7 +1364,11 @@ func mayberemovefile(s string) {
 func (b *Builder) fmtcmd(dir string, format string, args ...interface{}) string {
 	cmd := fmt.Sprintf(format, args...)
 	if dir != "" && dir != "/" {
-		cmd = strings.Replace(" "+cmd, " "+dir, " .", -1)[1:]
+		dot := " ."
+		if dir[len(dir)-1] == '/' {
+			dot += "/"
+		}
+		cmd = strings.Replace(" "+cmd, " "+dir, dot, -1)[1:]
 		if b.scriptDir != dir {
 			b.scriptDir = dir
 			cmd = "cd " + dir + "\n" + cmd